This is one of the most significant, dismaying, and underappreciated facts about the 2016 election: While a number of prominent Republicans refused to endorse Donald Trump, the only serving Republican member of Congress who backed Hillary Clinton was Rep. Richard L. Hanna (R-N.Y.) — and he was retiring. All the other Republicans for Clinton were either former officeholders, or people such as me who had never held office in the first place. Even those Republicans who knew how unfit President Trump was for office — a fact that daily becomes more evident — were unwilling to back the only candidate who could actually have defeated him.

For me, supporting Clinton wasn’t a close call: She was qualified and centrist, and her ethical issues, while real, faded into insignificance compared to Trump’s own. For those reasons, I had expected that many high-profile Republicans would campaign for Clinton. But it didn’t happen. Why?

Part of it undoubtedly was Clinton aversion and fatigue. But a lot had to do with the demonization of the Democrats and the tribal loyalty that have long been central to Republican identity — just as demonization of Republicans and a mirror-image tribal loyalty have been the case for Democrats. American politics has become so polarized, it is harder than ever for partisans to cross the aisle — even when the arguments for doing so are overwhelming.

Case in point: Sen. Bob Corker (R-Tenn.). He has called Trump “utterly untruthful” and has said that the president may set the country “on the path to World War III.” He has warned that the White House is becoming an “adult day-care center” because the president’s staff has “to figure out ways of controlling him.” And he has noted that Trump “lowers himself to such a low, low standard and debases our country.” Yet Corker says that if an election between Trump and Clinton were held again today, he would “probably” still vote for Trump. Huh?

More to the point, Corker won’t endorse the Democratic candidate to succeed him in the Senate — Phil Bredesen — as much as he would clearly like to. The former Tennessee governor is as centrist as a Democrat can be — he opposed a state income tax, cut the rolls of the state’s medical program for poor people, and is pro-gun rights. The Republican nominee is Rep. Marsha Blackburn (R-Tenn.), a typical Trumpite. She supported Trump’s crazy conspiracy-mongering about President Barack Obama’s birth certificate, his offensive desire to ban Muslims from coming to the United States, and his unrealistic plan to build a border wall. She also echoed Trump’s criticisms — catnip for racists — of African American NFL players who kneel during the playing of the national anthem.

But after Corker said he wouldn’t campaign against Bredesen, he was taken to the woodshed by Senate Majority Leader Mitch McConnell (R-Ky.). A chastened Corker appeared on TV on Sunday to say he is planning to support “this person” — “our nominee.” He couldn’t bring himself to even name Blackburn. It looked as if he were being held hostage. It was painful to watch.

I don’t see what’s holding Corker back from simply endorsing Bredesen, especially now that he is retiring. The same is true with other Republicans critical of Trump: They should be urging voters to support Democrats, because that is the only way to rein in a president who threatens the rule of law. But precious few do so, because of a pernicious combination of self-delusion and self-interest.

Republican politicians know their base loves Trump, and that this means openly challenging him is suicidal. They also fool themselves into thinking that they are serving the same old Republican Party that once stood for conservative ideas such as limited government, fiscal responsibility, international leadership, and free trade. They prefer to ignore what the GOP has actually become — a vehicle for waging a Trump-style culture war, feeding Trump’s egomania, and protecting Trump from accountability for his actions. With House and Senate leaders preventing consideration of a bill to protect special counsel Robert S. Mueller III, obstruction of justice has practically become a plank of the new Republican Party.

It’s not just Republican politicians falling into line. Big-money donors are just as pusillanimous, even though Congress has already given them their Christmas gift in the form of a budget-busting tax cut.

One of the few GOP donors who has switched to funding Democrats is the Boston hedge fund billionaire Seth Klarman. Once New England’s top Republican donor, he has given $222,000 to 78 Democrats running for Congress since 2016. “The Republicans in Congress have failed to hold the president accountable and have abandoned their historic beliefs and values,” Klarman told the Boston Globe. “For the good of the country, the Democrats must take back one or both houses of Congress.”

Klarman is right, and it’s a disgrace that more Republicans aren’t willing to put aside their partisanship for the good of the country.