In 1960, 5 percent of Republicans and 4 percent of Democrats said that they would feel “displeased” if their son or daughter married outside their political party. By 2010, those numbers had reached 49 percent and 33 percent. Republicans have been found to like Democrats less than they like people on welfare or gays and lesbians. Democrats dislike Republicans more than they dislike big business.

And this:

To test for political prejudice, Shanto Iyengar and Sean Westwood, political scientists at Stanford University, conducted a large-scale implicit association test with 2,000 adults. They found people’s political bias to be much larger than their racial bias. When Democrats see “joy,” it’s much easier for them to click on a corner that says “Democratic” and “good” than on one that says “Republican” and “good.”

To find out whether such attitudes predict behavior, Iyengar and Westwood undertook a follow-up study. They asked more than 1,000 people to look at the resumes of several high-school seniors and say which ones should be awarded a scholarship. Some of these resumes contained racial cues (“president of the African American Student Association”) while others had political ones (“president of the Young Republicans”).

Race mattered. African-American participants preferred the African-American candidates 73 percent to 27 percent. Whites showed a modest preference for African-American candidates, as well, though by a significantly smaller margin. But partisanship made a much bigger difference. Both Democrats and Republicans selected their in-party candidate about 80 percent of the time.