I think Paul Krugman is partly correct here (boldface mine):
The G.O.P., by contrast, is one branch of a monolithic structure, movement conservatism, with a rigid ideology — tax cuts for the rich above all else. Other branches of the structure include a captive media that parrots the party line every step of the way. Compare the coverage of recent political developments on Fox News with almost everywhere else; we’re talking North Korea levels of alternative reality.
And this monolithic structure — lavishly supported by a small number of very, very wealthy families — rewards, indeed insists on, absolute fealty. Furthermore, the structure has been in place for a long time: It has been 36 years since Reagan was elected, 22 years since the Gingrich takeover of Congress. What this means is that nearly all Republicans in today’s Congress are apparatchiks, political creatures with no higher principle beyond party loyalty.
But there’s a second problem. In a less polarized era, there were enough Republicans who wouldn’t lose everything if Nixon were impeached (or forced to resign). Liberal Republicans wouldn’t see every policy they liked want go up in smoke. Yes, they could lose majorities and policy might be pushed somewhat leftward.
But today, if you’re a Republican–which, as Krugman notes, means you’re a movement conservative–if Trump is impeached, you lose everything: anti-abortion federal judges, the ability to kill the ACA, all sorts of regulatory rules, the ability to massively tilt the electoral landscape in your favor. As importantly, most Republicans will gain nothing. Impeachment, and its consequences, would be a devastating blow to their agenda (mind you, this is a good thing).
So they won’t break until they are less likely to have their agenda move ahead with Trump than without him. And I’m not sure I see that happening anytime soon.