In the 1970s, about a quarter of all voters split their tickets, voting for presidential and congressional candidates of different parties. Now, not even one in 10 does so. The result:

In 2016, all 34 Senate elections and 400 of 435 U.S. House elections were won by the party winning the presidential election in the state or district.

In their analysis of negative partisanship, Iyengar and Krupenkin argue that voting on the basis of hostility has an unexpected consequence: It lessens pressure on the winner to be accountable to his or her supporters, effectively freeing winners to thumb their noses at many of the voters who put them in office:

When citizens’ support for a candidate stems primarily from their strong dislike for the opposing candidate, they are less subject to the logic of accountability. Their psychic satisfaction comes more from defeating and humiliating the out-group, and less from any performance or policy benefits that might accrue from the victory of the in-party. For this group of voters, candidates have every incentive to inflame partisan negativity, further entrenching affective polarization.

By this logic, Iyengar and Krupenkin argue that

If partisans care less about their own party’s performance and instead focus on their distrust of the opposition party, elected officials no longer need campaign on their own merits; instead, they have good reason to try even harder to denigrate the opposition.

In other words, Trump critics who anticipate a collapse of support when he fails to deliver on campaign promises are likely to be disappointed. According to Iyengar and Krupenkin,

The primal sense of “us against them” makes partisans fixate on the goal of defeating and even humiliating the opposition at all costs. This negativity bias in voting behavior undermines traditional theories of electoral accountability that rest on incumbents’ ability to deliver policy and performance benefits.

Candidates and incumbents, Iyengar and Krupenkin continue, “are less likely to be sanctioned for demonstrating incompetence, dishonesty and unethical behavior.”

To make their point, the authors cite Roy Moore, the Republican candidate for Senate in Alabama last December, pointing out that under partisan conditions “a candidate accused of molesting teenage children is able to attract 91 percent of the vote from his copartisans.”

What explains the extraordinary level of support for Moore, not only from Republican voters but also from the larger population of white Alabamians, at 68 percent, and conservatives, at 83 percent?

They can’t all have been happy with their candidate, but a partial answer can be found in a study published last month, “One Tribe to Bind Them All: How Our Social Group Attachments Strengthen Partisanship,” by Lilliana Mason and Julie Wronski, political scientists at the University of Maryland and the University of Mississippi.

The authors found that just as the Democratic Party is often described as a collection of identity groups, identity politics have also become a powerful force within the Republican Party. They argue, in fact, that “this effect is more powerful among Republicans than among Democrats, due to the general social homogeneity of the Republican Party,” with the result that “Republicans are more susceptible to identity-based politics.” For Republicans,

the general white, Christian conservative alliance with the party led to a far simpler categorization of who is in the group and who is outside of it.

Why would this lead to more willingness to vote for someone with as much baggage as Roy Moore? The convergence of voter identification with voters’ social identity — conservative, white, Christian — “makes in-party preference more powerful and out-party tolerance ever more difficult.”

Put another way, Republican doubts about Roy Moore were superseded by hatred of the Democratic Party.

There is no sign that the growth in negative polarization will slow down, much less reverse itself. The dominant incentive in politics right now is to capitalize on animosity to the opposition party.