Right now, there’s one thin reed preventing Republicans from doing anything they want starting in 2017: the filibuster. During the Obama administration, 
Republicans set a precedent of requiring 60 votes for essentially everything,1 and Democrats are now the beneficiaries of that. This will substantially limit what Donald Trump and a Republican Congress can do.
But will Republicans keep the filibuster? Democrats killed it for lower court appointments, and the same process can be used to kill it completely if Republicans have a mind to. Jonathan Bernstein cogitates on whether they’ll do it:
There will probably be 52 Republican senators in the new 115th Congress. Assuming the vice president would break any tie in favor of change, it would take 50 of those 52 to do away with the filibuster.
It’s by no means certain those votes are available.
For one thing, several senior Republicans, perhaps including John McCain and Lamar Alexander, may sincerely respect the traditions of the Senate and be reluctant to eliminate them. Moderate conservatives such as Susan Collins and Lisa Murkowski might feel the current rules give them more leverage than a Senate with pure majority-party rule.
….Cynical Republicans might, as Congress scholar Gregory Koger suggests, see some advantages in having some of their agenda obstructed by Democrats rather than being put in the position of having to pass some items….Far-sighted Republicans might worry about the longer-term consequences of giving a future Democratic majority….And every senator has an incentive to keep the filibuster because it strengthens their individual leverage, even if it reduces the ability of their party to get things done.
I think that pretty much covers it. There might be a few moderate Republicans unwilling to kill the filibuster—though the precedent of Obamacare makes me skeptical of that. At some point, even the “moderates” will convince themselves that Democrats have done something so dastardly that they have no choice but to end the obstruction. There might be a few cynical Republicans, who think they can do better in the next election if the conservative agenda fails and they can whip up the rubes about it. There might be a few far-sighted Republicans, though those seem in short supply these days. And there might be a few selfish Republicans.
So the continued existence of the filibuster relies on moderate, cynical, far-sighted, and selfish Republicans. The good news is that we only need three total. The bad news is that if the tea party and the Trumpkins get mad enough, there might not be three Republicans willing to buck them.
1Except for budget bills, which can be passed with a simple majority.
