WHAT HAPPENS WHEN...:

US midterm elections look odd to the European eye. If their dullness and predictability have a counterpart in the Old World, it would only be in elections to the European Parliament, which occur in a similar environment of low public interest and adverse conditions for the party in control of the national executive. Yet on both sides of the Atlantic, it is precisely this environment that brings to life an ancient creature that persistently refuses to go extinct: the political center. Centrist voters often find their moderate policy views unrepresented during prime-time performances of party polarization. Their big moment comes when elections are repeated off the beaten path.

At midterm - between presidential elections - when most of the nation falls back into apathy or learned patterns, centrists seize the opportunity to voice policy concerns and seek ideological balance. Both these objectives tend to hurt the party of the sitting president. While one may suspect agile independents are at work, survey evidence for all House elections from 1956 through 2018 reveals that pushback rather comes from the president's own ranks. In an average midterm election, the presidential party sees about 10 percent of its moderate partisan base defect to the opposition.

Moderate partisans (i.e., voters who identify with a particular political party) combine two features that encourage midterm strategizing. Their policy preferences are "sandwiched" between the two major platforms, giving them an incentive to vote against whichever party happened to get the upper hand by snatching the White House two years earlier. At the same time, moderate partisans are still partisans, meaning that they are biased arbiters of political competition. Ideally, they would like to see moderate policy being implemented by an administration of their own stripes. Voting against their allegiance at midterm allows them to remind their party that the next presidential race will be won in the center. The evidence suggests that both these motivations--balancing policy and voicing a warning--need to come together to produce measurable impact.

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