The reason is simple: Partisan polarization. However bad Trump is for the Republican Party, GOP lawmakers fear abandoning him will cause even more problems with their base.

On Tuesday, Republican Sen. Mark Kirk, who is fighting to keep his Illinois seat, rescinded his Trump endorsement after the celebrity businessman claimed that Judge Gonzalo Curiel, an American born in Indiana, is unfit to consider a lawsuit against the now-shuttered Trump University. Arizona Sen. Jeff Flake and Wisconsin Gov. Scott Walker also cited the controversy in refusing to back Trump.

But for each Republican holding off, many more have tried to distance themselves from Trump’s comments while still supporting his candidacy.

“It’s time to quit attacking various people you competed with or various minority groups in the country and get on message,” Senate Minority Leader Mitch McConnell said Tuesday.

House Speaker Paul Ryan on Tuesday called Trump’s attacks on Curiel racist and “absolutely unacceptable.”

But Ryan didn’t mean that. He may disapprove of Trump’s attack on Curiel, but he accepts it by still backing the candidate. The same goes for McConnell and for Republicans like Florida Sen. Marco Rubio, who bashed Trump’s comments on Monday and then raised money for him on Wednesday.

Republican Sen. Lindsey Graham of South Carolina on Monday suggested his colleagues use the Curiel controversy to ditch Trump. “If anybody was looking for an off-ramp, this is probably it,” Graham said. “There’ll come a time when the love of country will trump hatred of Hillary.”

Maybe not. Sen. Roger Wicker of Mississippi, the head of Senate Republican’s campaign arm said he does not expect other GOP candidates to join Kirk in renouncing Trump.

Wicker is probably right. Most Republicans will stick with Trump even as they fret he will cost them their jobs. Senior GOP lawmakers can’t quit a candidate they have said is no conservative and might increase the odds of nuclear war. Why? Because Republicans care less about those things than about their team winning. Partisanship prevails.

Republicans have signaled this for months. GOP lawmakers spent the winter and early spring saying they would support their party’s nominee. Now that they are stuck with Donald Trump, they offer a new take: They prefer him to Hillary.

“This is the choice,” McConnell said on Sunday, agreeing with an interviewer that Trump is the “lesser of two evils.”

Republican officials stick with Trump for the same reason that most Bernie Sanders backers will vote for Hillary Clinton in November. A decades-long rise in partisan identification has left American voters in hostile camps made up of people who believe they are competing in a zero-sum game. They want their side to win. Progressive Bernie Sanders voters may not love Clinton, but they like Republicans less. If Clinton wins, conservatives think they lose.

An extensive 2014 Pew study of public perceptions illustrates this situation. According to the study, the percentage of Americans who have what Pew refers to as consistently conservative or consistently liberal opinions has doubled over the past two decades, from 10% to 21%.

Pew also found that ideological overlap between the two major political parties has effectively disappeared: In 2014, 92% of Republicans were to the right of the median Democrat, versus 64% in 1994. In 2014, 94% of Democrats were to the left of the median Republican, versus 70% two decades ago.

As Democrats and Republicans drifted away from each other ideologically, animosity between the two groups has only grown. The share of Republicans and Democrats with a highly negative view of the opposing party has more than doubled since 1994. Now, two-thirds of consistently conservative Republicans think Democrats’ policies threaten the nation’s well-being. Half of liberal Democrats say the same of Republican policies.

Americans are divided socially, regionally, and culturally. Nearly two-thirds of consistent conservative voters and about half of consistent liberals said most of their close friends share their politics. Pew found that 50% of conservatives said it was “important to them to live in a place where most people share their political views,” along with 35% of liberals.

These figures explain why lawmakers like Ryan and McConnell back Trump while both living former GOP presidents and the 2012 Republican presidential nominee Mitt Romney have not endorsed him. Republican primary voters picked Trump. To dismiss him, Republican lawmakers would have to tell those voters they are wrong. Why won’t Sens. John McCain of Arizona or Pat Toomey of Pennsylvania or Kelly Ayotte of New Hampshire ditch Trump, despite facing tough reelection fights? Those senators have to run in states where Trump won primaries, and they need his backers’ votes. Retired politicians don’t.

For Trump voters, along with many on the left, politics is identity, as much as a person’s religion or hometown. According to Pew, 23% of consistent Democrats would be unhappy if an immediate family member married a Republican, while 30% of consistent conservatives would be peeved if a family member married a Democrat.

Republican leaders can’t ditch Trump. Their most-committed supporters would see it as a betrayal. If you can’t handle your daughter marrying a Democrat, you won’t like your senator ditching the party’s nominee.

Pressed to explain their endorsements, Republican lawmakers repeat that Trump is better than the alternative. Most don’t elaborate. They stand by the home-team.

Sen. Bob Corker, a Tennessee Republican mentioned as a potential Trump vice presidential pick, refused three times on Tuesday to say if Trump is “fit” to be president. But Corker still supports Trump.

While campaigning against Trump, Sen. Marco Rubio called him an “embarrassment,” a “con artist,” “a person that has no ideas of any substance on the important issues,” and “an erratic individual” who should not be handed “the nuclear codes.” He compared Trump to North Korean dictator Kim Jong-un.

But despite indulging himself recently by noting: “I warned you this is what was going to happen,” Rubio, who clearly retains presidential ambitions, still supports Trump.

Why? “Because I don’t want Hillary Clinton to be president,” Rubio said recently on CNN. He didn’t explain further. He didn’t have to.