There are four main reasons why congressional Republicans are hesitant to take on Trump. The first is simple tribal loyalty. He identifies as a Republican, he ran as a Republican, and he was elected as a Republican. And—the Tea Party excepted—the Republican Party of the last several decades has been very hesitant to move against other Republicans. You can argue that a similar dynamic exists among Democrats, and you would be correct. But that doesn’t invalidate it as a reason why congressional Republicans hold back.
The second is the electoral position of the Republican party. The Republican agenda is not terribly popular in the United States. The core elements—massive tax cuts for the wealthy, slashing social security, medicare, education, and similar discretionary spending, and repealing Obamacare—all poll at between 30% and 35% popularity. In a democracy, these are not policies that are naturally going to prevail. Yet these are the policies that congressional Republicans are sworn to uphold. Through a combination of luck and a manipulation of process they are actually in a position to possibly pass some or most of this over the preferences of 60 to 70% of the country. They are trying super hard not to mess this up.
The third reason is the tendency on part of many who are by temperament drawn to the Republican side to feel fundamentally that respect and deference to authority belong in the proper order of things. As such they are far more hesitant to challenge what they see as legitimate the authority than those who by temperament are drawn to more liberal views. So for these three reasons congressional Republicans a lot less likely to want to challenge President Trump.
So loyalty to team, fragility of position, and temperament are three reasons why we should not expect congressional Republicans to check Trump’s actions in the White House, even when they are blatantly unconstitutional. Congressional Republicans realize that it was only on Trumps message of economic populism—and not on their message of more handouts for the rich—that they are in a position to pass their agenda. If they dump Trump, they’ve lost their cover for pushing deeply unpopular policies behind the distraction that is the Donald.