Jennifer Rubin attempts to explain "what stops Republicans from behaving rationally:"
First, unlike Senate and House Republicans during Watergate, there are few genuine leaders of principle whose sense of propriety is offended by Trump. The moral and intellectual quality of the current crew of Republicans pales in comparison to the type of Republicans who finally told Richard Nixon the jig was up. Sen. Barry Goldwater (R-Ariz.), House Minority Leader John Rhodes (R-Ariz.) and Senate Minority Leader Hugh Scott (R-Pa.), who went to the White House, have few if any equals in today’s House and Senate.
Second, elected Republicans by and large cower in the shadow of Fox Non-News hosts, talk-radio opportunists and right-wing interest groups. They fear noticeable distancing from Trump will prompt the vultures of the right to swoop down up them, leaving only bones behind. So long as the characters who populate the right stick with Trump, elected Republicans, sadly, won’t lead.
Krugman, channeling Nate Silver, sees lopsided election wins in Republican districts and epistemic closure as the root causes:
But mightn’t even Republican voters turn on you if you seem too slavish to an obviously corrupt leadership? Well, where would those voters get such an idea? For all practical purposes, Republican primary voters get their news from wholly partisan media, which quite simply present a picture of the world that bears no resemblance to what independent sources are saying. Even though most Republicans in DC probably know better, their self-interest says to pretend to believe the official line.
So if you’re Representative Bomfog from a red state, your entire career depends on being an apparatchik willing to do and say anything the regime demands. Suggestions that the president’s men, and maybe the man himself, is in collusion with a foreign power? Fake news! Firing the FBI director in an obvious obstruction of justice? Let’s make excuses! Analyses suggesting that your bill will cause mass suffering? Never mind. Party loyalty is all — even if it demands humiliating displays of obsequious deference.
The one thing that might cause Rs to turn on Trump would be the more or less certain prospect of a wave election so massive that even very safe seats get lost. And at the rate things are going, that could happen. But if it does, it will be nothing like a normal political process; it will be more like a revolution within the GOP, a regime change that would shatter the party establishment.
Meanwhile, Kevin Baker has a good argument that Trump isn't Nixon, he's Jackson, but without the skill or strategy.