Unexpected events like terrorist attacks, natural disasters, political scandals or even sports results have impact on voters’ moods and policy preferences and thus unravel important political consequences. This paper examines how, in a context of electoral party competition, the proposals of political parties adjust to respond to such sudden changes in voters’ preferences.

When an exogenous shock affects the policy preferences of voters, the equilibrium that competing political parties had attained prior to the shock is upset. Changes in voters’ preferences change voting decisions and parties must realign their policy positions in order to react optimally. The magnitude of the policy shifts needed by each party depend on the magnitude of the shift produced on the voters’ preferences, and thus, on the intensity of the shock, but also on the optimal reaction to opponents’ re-adjustments. Changes in voter’s preferences favour some parties and damage others. Hence, the optimal policy revisions that follow after a preference shock are asymmetric across parties.

An exogenous shock can affect the voters’ policy preferences and their voting behavior in two different ways. On the one hand, a shock might shift the voters’ ideal points on a given issue. For example, in the case of a terrorist attack it is plausible to think that all voters shift their preference towards an increase in national security. Thus, in the policy dimension affected by the shock, the ideal points of all voters change and they all change in the same direction. On the other hand, a shock may change the balance between the different policy dimensions. Voters are likely to shift their attention to the policy dimension affected by the shock, and regard it as more important and relevant. Thus the salience of a previously unimportant policy dimension may increase, turning it into the dominating issue after the shock.

A preference shock benefits some parties and hurts others. When a shock drives the ideal policy of all voters closer to the ideal policy of a given party, then this party will enjoy an electoral advantage because earning each vote becomes cheaper in ideological cost: to attract votes after the shock this party must make policy proposals that are closer to his ideal points than the policies required prior to the shock. At the same time, for a party with opposed views, the shock causes a disadvantage because the shock drives the voters’ ideal points away from the ideal policy of the party. Each single vote becomes more expensive in ideological cost after the shock: to attract votes after the shock requires policy choices further away from his ideal points than those proposed before the shock. Consequently, the parties’ policy adjustment as a reaction to a voters’ preference shift are asymmetric.

A main focus of our interest are the consequences of a preference shock that changes the relative salience of issues; that is, situations where voters’ attention shifts unexpectedly and an issue that was previously disregarded suddenly becomes the relevant dimension of political competition. When voters turn their attention to a new issue their decisions are based on their evaluation of the parties’ policies on this new salient issue, and parties must strategically choose policy positions on the new relevant issue. If the two parties have different ideal points in this new salient issue, then they will move to compete in the new dimension, aiming to attract the voters with ideal points located between their opposed ideal points. Party competition in the new issue may face a symmetric distribution of voters, in which case both parties will optimally moderate a bit their positions. But, the new salient issue may be one where voters’ views are closer to one of the two parties; thus one party is favored by the proximity of the median voter to its ideal point and the other party is damaged by the larger distance between the median voter and its ideal point. In this case, the optimal policy choices of parties would imply asymmetric moves from their ideal points. This is the scenario where the shock delivers the most interesting consequences while maintaining the electoral competition among the initial parties. There is one last possible scenario: If both parties hold similar positions regarding the new issue, i.e. both parties share the same ideal point in the new salient issue, then new parties may find it profitable to enter the electoral competition with proposals that confront the one shared by the two old parties, so that the change on issue salience might break up the stability of the party system. We focus our analysis on the case with asymmetric voters’ preferences. In particular, we consider the effects of a polarized preference distribution.

To analyze these phenomena we construct a model of two party electoral competition with policy motivated parties and sincere voters. We assume that there is a decisive voter whose ideal point is unknown to the parties and parties have beliefs about it that are common and common knowledge. In this model we introduce two assumptions that relate closely to the two effects described above: an exogenous shock produces an increase of the salience of a given policy dimension and in this dimension the voters’ preferences are polarized. We characterize the equilibrium policy choices of two parties that compete in an election both before and after the shock takes place, and compare the different outcomes that arise.

In equilibrium, parties that are policy motivated choose policies that are moderate compared to their ideal points. If they compete on a dimension with a uniform distribution of the voters’ preferences then the policies chosen by the parties are symmetric with respect to the expected location of the ideal point of the decisive voter (the median voter). These results are in line with the ones described in Calvert (1985) and Wittman (1977, 1983). If parties compete on a dimension with an asymmetric distribution of the voters’ preferences then the policies chosen by the parties will moderate following the direction of the asymmetry of the distribution of the voters’ preferences and the magnitude of the adjustment will be different for each party. In fact, if the distribution of the voters’ ideal points is biased to one side of the policy space then the ideal point of the decisive voter will be relatively closer to the ideal point of one of the two parties, and away from the ideal point of the other party. This implies that one party benefits from the shock while the other party suffers damage. The party that is damaged by the shock needs a greater adjustment in its policy proposal in order to remain competitive in the electoral contest, while the party favored by the shock needs a smaller change, only to react to his opponent change of strategy. Thus, compared to the policies chosen prior to the shock, the policy choice of the party disfavored by the shock is more moderate, while the policy of the party favored by the shock can be less moderate (closer to the party’s ideal point).

The equilibrium policies chosen by the parties depend on the relative salience that the voters assign to each of the issues. The more salient an issue the more moderate are the parties’ policy choices on that issue. In particular, if only one issue is salient, that is, under the assumption that voters base their voting decision only on the policy proposals regarding the issue they consider most important then the equilibrium policies on the salient issue coincide with the outcomes of a one-dimensional model, while the equilibrium policies on the non salient issue coincide with the parties’ ideal points. Indeed, there is no reason for the parties to compromise their policy positions on an issue if voters are not paying attention to it.

When a shock changes the issue that voters consider salient, both parties react by a substantial moderation of their choices, moving their policies towards the ideal point of the expected median voter, in the newly relevant dimension, while they turn radical, towards their ideal points in the dimension that is no longer salient. More interestingly, if the shock increases the salience of an issue for which the voters’ preferences are polarized in an asymmetric way the in equilibrium parties may react by choosing divergent policies. The party that is damaged by the shock reacts by moderating a lot its policy, while the party favored by the shock does not need to moderate its policy so much. However, now the outcomes after the shock have to be compared to the parties’ ideal points on that dimension, since this was their optimal choice prior to the shock, when the affected issue was not salient. We also find that if the distribution of voters’ preferences after the shock are very asymmetric in equilibrium parties do not moderate their policies at all. Instead they choose to implement their ideal points. We offer a characterization of the conditions on the parameter values of the voters’ distribution that produce each type of equilibrium outcomes.

The asymmetry in policy reactions that follows whenever a shock affects the voters’ relative issue salience, is consistent with the empirical evidence provided by Plümper and Epifanio (2015). They analyze the changes in antiterrorist policies implemented by incumbents of different countries after a terrorist attack. They find that the intensification of antiterrorist policy is greater for leftist incumbents than for rightist incumbents. The formal arguments developed here offer a theoretical explanation for the empirical observation provided by Plümper and Epifanio.

Our results also point out that shocks that induce changes in relative issue salience may affect the electoral balance between the two competing parties. By favoring one party over the other, the shock imposes different electoral costs to the parties: it makes electoral competition much lighter for one party and much more costly for the other. Implementing a large policy shift may be very costly for a party, because compromising ideological principles may cause internal frictions among the different factions within the party and may end up destroying the party’s internal equilibrium. At the same time, when a shock brings to salience a new policy issue, if the positions of established parties on the new issue are too similar, the shock might induce the entry of new parties and reconfiguring the political landscape.

We illustrate this argument with a discussion of the recent political upheaval in Catalonia and the United Kingdom. In 2010 high court decisions in Spain boosted the salience of the independence issue among Catalan voters’. As a consequence the two main political parties faced demands for drastic changes in their policy proposals and suffered major crisis. The Catalan party system of more than thirty years has been knocked down. The Brexit referendum is another shock that has turned on the salience of a previously disregarded issue and brought about a major crisis in the political party system.

The rest of the paper is organized as follows. In the next section we argue that unexpected events that change the relative issue salience and may also polarize the voters’ preferences are a relevant fact and we review the empirical evidence that supports this claim. Section 3 turns to the theoretical model. In Sect.  4 we examine the effects of an increase of salience of an issue where voters’ preferences are polarized on the parties’ equilibrium policy choices. In Sect. 5 we illustrate how changes in issue salience that require a large policy shift cause instability on party’s internal consistency. Section 6 concludes with some final remarks.

It is commonly argued that the policy preferences of the voters are conditioned and even determined by the policy stands of the parties during campaigns, and by the policies implemented by incumbents. Indeed, it is in the interest of the parties to try to manipulate voters’ opinions through the media, and electoral campaigns. This priming phenomenon is discussed in numerous experimental and empirical studies in psychology, political psychology and political science including Bartels (2006), Iyengar (1990), Iyengar and Kinder (1987), Iyengar et al. (1982), Kahneman and Tversky (1979, 1981, 1984), Krosnick and Kinder (1990), Sheafer and Weimann (2005). For a critique, see also Lenz (2009). Similarly, the literature on policy and policy preference responsiveness (Barbera et al. 2019; Clinton 2006; Kastellec et al. 2015) shows that policy agendas are mainly driven by the political parties, and finds weak empirical support for the claim that politicians are responsive to the general public.

However, public opinion and voter’s preferences are also shaped by dramatic, unexpected events that are exogenous to parties’ actions. This has long been recognized in the literature on agenda-setting, that identifies “focusing events” as a leading cause for changes in public opinion and shifts in the political debate (Bangartner and Jones 1993; Birkland 1998; Kingdon 1995; Walker 1977). According to Birkland (2017) p. 74: “ Focusing events are sudden, relatively rare events that spark intense media and public attention because of their sheer magnitude or, sometimes, because of the harm they reveal. Focusing events thus attract attention to issues that may have been relatively dormant. Examples of focusing events include terrorist attacks (September 11, 2001 was, certainly, a focusing event), airplane accidents, industrial accidents such as factory fires or oil spills, large protest rallies or marches, scandals in government, and everyday events that gain attention because of some special feature of the event. Two examples of the latter are the alleged beating of motorist Rodney King by the Los Angeles Police Department in the early 1990s and O. J. Simpson’s murder trial in 1995; the Rodney King incident was noteworthy because, unlike most such incidents, the event was caught on videotape, while the Simpson trial was noteworthy because of the fame of the defendant. Focusing events can lead groups, government leaders, policy entrepreneurs, the news media, or members of the public to pay attention to new problems or pay greater attention to existing but dormant (in terms of their standing on the agenda) problems, and, potentially, can lead to a search for solutions in the wake of perceived policy failure.”

There is a substantial empirical literature documenting the effects of dramatic, extraordinary events on public opinion and election outcomes. Studies examine the consequences of nuclear power accidents (van der Brug 2001), natural disaster and accidents involving loss of life (Birkland 1998; Slovic et al. 1984), the assassinations of important social or political figures such as Martin Luther King (Hofstetter (1969)) or terrorism attacks. Balcells (2018) examine eight violent attacks perpetrated between 1989 and 1997 by the a Basque terrorist organization Euskadi Ta Askatasuna (ETA). They measure the effect of attacks on public opinion surveys that were being fielded when the attacks occurred to estimate the causal effect of terrorist violence on individuals’ intent to participate in elections as well as on professed support for the incumbent party. They find that terrorist attacks significantly increase individuals’ intent to participate in a future election (with a greater impact in attacks against civilians) but find no evidence that the attacks change support for the incumbent party. In general, identifying the causal effect of exogenous shocks on voters’ preferences is not a simple exercise. Muñoz et al. (2019) review studies that exploit the role of exogenous shocks during the fieldwork of public opinions surveys taking them as natural experiments to explore causal estimates, and discuss the general strengths and limitations of this identification strategy.

The terrorist attacks of March 2004 in Madrid and their impact in the subsequent election to the Spanish Congress is an interesting case. On March 11, only three days before the election, a major Islamist terrorist attack killed 193 people and injured around 2000. Polls taken before the bombings showed a clear advantage of the incumbent conservative party over the socialist party. However the actual vote delivered a clear victory to the socialist party. The important difference between the results of the polls taken before the attack and the election results seemed to indicate that the voters might have changed their vote intention as a result of the bombings. Indeed, the conservative party, incumbent at that time, might have been considered by many as responsible of the attack, because of the government decision to support the US and UK in Iraq sending Spanish military forces just a few months prior to the election. Instead, in his electoral platform, the socialist leader had promised to bring the troops back home in case of victory. Papers that analyze the relationship between the attacks and the election results on the Spanish congress using post election surveys deliver inconclusive results. While the results of Bali (2007) seem to imply that the bombings were decisive, Lago and Montero (2005) find that they had no effect. Montalvo (2011, 2012) takes a different approach; instead of survey responses he examines actual voting results. He compares the outcome of the vote on election day with a control group of individuals that voted before the terrorist attacks: residents abroad had cast their vote before March 7 at a Spanish consulate or by post, thus they voted before the bombing took place. It turns out that the results of the vote for this control group showed an advantage to the conservative party, as predicted by the polls, and their turnout participation level was consistent with the normal trend of previous elections. Instead, the votes cast on election day, after the bombings, reversed the results predicted by the polls and showed a significant advantage for the socialist party over the conservative party and, in addition, a significant increase in turnout compared to previous congressional elections. Hence, this natural experiment approach provides evidence that the bombings changed the voters’ political views and induced voters to turnout in larger numbers. The change of political views in this case was a demand to reconsider international alliances, and the higher turnout arisen from the enhanced political engagement by citizens when the relevance of the policy dimension affected by the shock increased.

Overall, the literature provides strong arguments and substantial empirical evidence to support the claim that, shocks that induce changes in voters’ preferences and the relative salience of issues do occur, and that they are exogenous to the actions of political parties. And therefore, they have major political consequences in re-aligning policy proposals and party systems.