They are hostages and we are all hostages to the hardcore <40% minority of Trump fans, as the us vs. them division hardens. I just ran across this from March 21 @ WaPo, the sad reality is right here, they can't afford to lose the Trump fans and all the Trump fans care about is supporting Trump, more than they care about any particular GOP thing:

A GOP senator’s (Bob Corker) remarkable admission about Trump and Mueller by Greg Sargent

[....] Republicans appear increasingly dug into their position. Their stance is that of course Mueller should be allowed to finish his investigation, but they will not act legislatively to protect the probe, because this is not at all necessary, as Trump would never dream of taking action against it, since he would face severe consequences that Republicans will not enunciate in advance.

But a Republican lawmaker has just given away the real game behind this carefully crafted straddle. Sen. Bob Corker (Tenn.) was pressed by the Washington Examiner on why Republicans are hesitant to protect Mueller, and this is what happened:

Republicans in Congress are hesitant to antagonize President Trump ahead of ahead of difficult midterm elections, wary of sparking a backlash from a committed grassroots base more loyal to the White House.

Amid sky-high Democratic enthusiasm and a developing “blue wave,” Republicans can’t afford a war with Trump that depresses GOP turnout. Republicans might be worried about Trump’s attacks on special counsel Robert Mueller, but they are reluctant to push back, much less support legislation to curtail the president’s ability to fire Mueller and sideline the federal probe …

“The president is, as you know — you’ve seen his numbers among the Republican base — it’s very strong. It’s more than strong, it’s tribal in nature,” said Sen. Bob Corker, R-Tenn., who decided to retire when his second term concludes at year’s end, after periodically sparring with Trump.*

“People who tell me, who are out on trail, say, look, people don’t ask about issues anymore. They don’t care about issues. They want to know if you’re with Trump or not,” Corker added.

This is a candid glimpse from a leading GOP lawmaker into what’s really driving the Republican straddle on Mueller. As I’ve noted, Trump’s attacks on Mueller are putting Republicans in a tough spot. The educated swing voters who are driving the Democratic anti-Trump resurgence support the Mueller probe and may vote to oust Republicans who won’t check Trump’s excesses. But Trump’s attacks probably rally GOP base voters, large percentages of whom see the Mueller probe as a witch hunt, making it harder for GOP lawmakers to protect that investigation [....]