In the GOP, by contrast, it’s easy to imagine. Republican leaders may not be bigots. But they often tolerate bigotry. In 2012, Mitt Romney gratefully accepted Donald Trump’s endorsement, even though Trump had spent the previous year implying that Obama had only gotten into college because he was black, demanding Obama’s birth certificate, and suggesting that “maybe it says he is a Muslim.” In 2011, GOP presidential candidate Herman Cain said that, if elected, he would not appoint Muslims to his administration. Last year, Ben Carson said America should not elect a Muslim president. Trump himself infamously called for temporarily banning Muslims from entering the United States.

For a Democratic presidential candidate, such statements would be political suicide. But in today’s GOP, they are not. Many of Cain, Carson, and Trump’s rivals distanced themselves from those views. But with the honorable exception of Lindsey Graham, no rival said the comments were disqualifying. As far as I’m aware, no rival except Graham even demanded an apology. As for ordinary GOP voters, they largely voiced their assent. A March Rasmussen poll found that 71 percent of likely Republican voters (as opposed to only 34 percent of their Democratic counterparts) back Trump’s Muslim ban. A Public Policy Polling survey from last fall found that a majority of likely Republican primary voters in Iowa either favored criminalizing Islam or weren’t sure. When asked last November by The Economist and YouGov whether they agreed with Trump’s statement that, “When Mexico sends its people, they’re not sending their best. They’re sending people that have lots of problems. They’re bringing drugs. They’re bringing crime. They’re rapists,” 63 percent of Republicans (compared with only 23 percent of Democrats) said they agreed.

It was not inevitable that anti-bigotry would become a partisan dividing line. In 1964, a majority of both House Democrats and House Republicans voted for the Civil Rights Act. (Republicans by a slightly higher margin.) In the mid-1990s, leading Democrats and Republicans both opposed gay marriage. In 2000, George W. Bush spoke out against the fact that Arab Americans were being “racially profiled.” As recently as 2008, neither John McCain nor Barack Obama demonized Mexican immigrants.

What’s changed? The divergence began when Democratic presidents signed civil-rights legislation in the 1960s and Republicans responded by taking the white South. As a result, African Americans became a key Democratic voting bloc while the GOP remained overwhelmingly white. There’s some evidence that, in the years prior to Obama’s election, this racial polarization was easing, with younger African Americans showing less loyalty to the Democratic Party and Bill Clinton winning a significant chunk of the white South. But as UCLA political scientists Michael Tesler and David O. Sears have documented, Obama’s election supercharged the party’s racial identities. Since 2004, Americans who exhibit higher levels of “racial resentment” have moved toward the GOP and those who exhibit lower levels have moved toward the Democrats.