Well...ostensibly, there's the minority and "rogue" elements in Congress, the Courts and the States.  Throw in any number of organizations that are willing to spend their cash on lawsuits. 
 Assuming the looming onslaught of change in 2018 actually happens, there will be that, as well.

If nothing else, the Republicans proved out during the early years of Obama how a party can put checks on Trump.  All the Democrats have to do is figure out how to coalesce and make it happen...which it seems they're struggling to do in a lot of respects.