The answer to that question is complicated, and it goes well beyond the fact that Mr. Trump is advancing certain policies they believe in. The case for Republicans casting a vote for Mr. Trump in 2016 on those grounds is defensible, though I made a different choice. But here’s what’s also true: Republicans could support Mr. Trump’s agenda while simultaneously condemning his corrupt behavior. Yet the vast majority of them refuse to do so. Something more is going on here. In order to find out what, I spoke to several former Republican officials and aides, who all requested anonymity in order t o speak freely.

Understanding the close compact between Mr. Trump and the Republican Party starts with acknowledging the false hope many establishment Republicans placed in the shady real estate mogul as he rose to power. They misdiagnosed the individual they were dealing with, assuming that Mr. Trump would “grow in office” and that they, the “adults in the room,” would be able to control and contain him. At the outset of this unholy alliance, they were convinced they would change Mr. Trump more than Mr. Trump would change them. But the transformation turned out to be in them, not him.

Nor should one underestimate the role self-interest plays in the Trump-Republican relationship. “All people in elective politics are interested in self-preservation,” a Republican who served in the Senate told me. “Any negative comment by a Republican in Congress about the president would invite trouble in the next election either in the form of opposition in the next primary or lack of support in the general.”

“Until Republicans feel their existence is threatened by Trump’s mendacity and corruption, most will be blind to it,” a former Republican presidential aide told me. “That isn’t to say there won’t be leading indicators of potential moral restoration,” he said, “but in the meantime, the cowed stay in line like the trophies of a bully.”

“This storm has more staying power than most,” he added, ominously.

There’s also this to factor in: For many Republican members of Congress, the president is more popular among Republican voters in their districts and states than they are. Even in the districts of moderate Republicans, Trumpist true-believers are the most vocal supporters of the party, the ones who do the essential grass-roots work. So to go crosswise of a sitting president of one’s own party invites political trouble, which most politicians seek to avoid. That is especially true when it comes to Mr. Trump, who specializes in savaging Republicans — and everyone else, for that matter — who dare to criticize him.

“They don’t want to get bludgeoned,” a person who served in Congress told me. “Their mind is looking for a rationale for not having to do it.” In addition, this person added, Mr. Trump has “conditioned people in the base so much so that it’s just ‘us versus them’ and that if you give an inch on him, you’re just giving the other side what they want. You’ve made yourself a ‘useful idiot.’”

“It’s easier to stay with the tribe,” this individual told me. “It’s easier to stay with the team.” Even when the leader of the team is thoroughly corrupt.