In a recent op ed, Harvard law professor Cass Sunstein points out that “partyism” – prejudice against supporters of the opposing political party – is on the rise in America:

If you are a Democrat, would you marry a Republican? Would you be upset if your sister did?

Researchers have long asked such questions about race, and have found that along important dimensions, racial prejudice is decreasing. At the same time, party prejudice in the U.S. has jumped, infecting not only politics but also decisions about dating, marriage and hiring. By some measures, “partyism” now exceeds racial prejudice — which helps explain the intensity of some midterm election campaigns.

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.

Increasingly, we assume that supporters of the rival political party are not just misguided about political issues, but also untrustworthy or malevolent people in general. Some of this hostility is due to “rational irrationality,” the tendency of people to make poorly reasoned knee-jerk judgments about political issues. It is closely related to the broader problem of rational political ignorance, which results in an electorate that often ignores the complexities of political issues, and tends to assume that those who disagree with their views must be evil, selfish, or stupid.

The growth of partyism in recent years is likely related to the rising polarization between the two parties, with conservatives increasingly “sorting” into the GOP and liberals into the Democratic Party. In the 1960s and 1970s, the correlation beween partisan affiliation, ideology and social values was much weaker than it is today. There were still many liberal Republicans and conservative Democrats. Today, such exotic beasts are few and far between. As a result, partisan divisions are far more closely correlated with wide divergences in values and ideology. That trend likely exacerbates partisan hostility. Unlike in 1960, if you meet a supporter of the opposing party today, he or she is likely to have widely divergent values from your own.

Whatever the cause, the growth of “partyism” is a dangerous trend. It makes partisans even less willing to be open-minded about ideas opposed to their own than they would be otherwise.

“Partyism” may also reinforce the tendency of political partisans to tolerate behavior from their own party leaders that they would never accept from those of the opposing party. The more you believe that the opposing party and its supporters are malevolent or stupid, the more reluctant you will be to give them ammunition by criticizing the bad behavior of your own party.

That tendency may help explain the decline of the antiwar movement in the age of President Obama, a phenomenon recently analyzed by David Boaz and Dana Milbank of the Washington Post. With the campaign against ISIS, Obama is now waging his second war without congressional authorization. He has reneged on promises to close down the Guantanamo detention facility for suspected terrorists, and used drone warfare far more aggressively than George W. Bush did.

If a Republican president had behaved the same way, antiwar liberals would have been up in arms. In fairness, some prominent liberal legal scholars have denounced Obama’s actions. But most grassroots progressives have either supported the president, engaged in only muted protest of his wars, or kept quiet about the subject altogether. Milbank suggests that many liberals have given Obama a pass because “they view him as a lesser evil than the Republican alternative.” Anti-Obama leftists whom he interviewed also believe that partisan bias is a major factor:

“If George W. Bush were launching wars with Congress out of town, oh, it would be flooded,” longtime liberal activist David Swanson said, looking across mostly empty Pennsylvania Avenue [during a tiny antiwar demonstration,] “They would be screaming.”

Swanson, who voted for Obama in 2008 before switching to the Green Party, said liberals who condemned Bush look the other way when Obama does the same thing because “he’s more eloquent, he’s more intelligent, he’s African American, he bills himself as a constitutional scholar.”

Left-wing Democrats are not the only ones who tolerate behavior from their own party that they condemn when the opposition does it. Many Republicans tolerated or actively supported George W. Bush’s massive expansion of federal spending and regulation, including the creation of the biggest new entitlement program since the 1960s. As their reaction to Obamacare suggests, they would never have tolerated similar behavior by a Democratic president.

Partisan prejudice of this type can sometimes be beneficial. It occasionally allows a president of one party to adopt valuable reforms that the other party would be unable to push through because it would encounter too much resistance. For example, Bill Clinton was able to push through welfare reform and free trade agreements much more easily than a GOP president. His status as a popular Democrat quiesced much of the liberal opposition that these policies would otherwise have encountered.

In general, however, partyism is more likely to be pernicious than useful. By adding another layer of bias to the already severe problem of political ignorance and irrationality, it further reduces the quality of public opinion on political issues.