Over the past two decades, Republicans have moved further to the right than Democrats have to the left—from which one might infer that Republicans’ negative feelings about the Democratic Party have grown more than Democrats’ negative feeling about the Republican Party. But that is not the case. According to David Kimball, Bryce Summary and Eric Vorst, a team of researchers at the University of Missouri, the level of partisan antipathy to the opposing party is about the same in both cases and has risen about as much since the late 1970s.

This trend—increasing mutual antipathy—helps explain the distinctive and distressing tone of our national politics. Today, nearly half of all partisan voters say that they fear the other party’s presidential candidate, and nearly two-thirds express anger toward that candidate.

Mr. Abramowitz finds a strong link between partisan preferences and ideological commitments. In 1972, only 20% of voters expressed a strong ideological preference for one party. By 2008, their share had risen to nearly half the electorate, and it remained at that level in 2012.