Even though the Republicans control the House and the Senate, as well as the Presidency, that doesn't mean that there are not differences of opinion among the party. To the contrary, Paul Ryan didn't even endorse Trump as a candidate. George W. Bush didn't endorse anyone either. The democrats will work with the Republicans to stop Trump.
The Democrats are going to be basically useless against Trump and his Republican puppets. Those Republicans have already shown that they will do whatever Trump asks of them, and they outnumber the Democrats greatly. The best the Democrats can do is persist and hang their heads high, in hopes of a drastic change in a couple of years.
No, the Democrats cannot keep Donald Trump in check. Today's Democrats care more about staying in power and pleasing their wealthy donors than they are in changing our country for the better or stopping unwanted events from taking place. They will continue to play lip service to minorities to gain votes.
The Democrats currently have very little power in the United States. Republicans control both the House and Senate, along with the presidency. Therefore, if Congress is going to keep Donald Trump in check, it needs to happen with help from Republicans. During the election, it became clear that many Republicans did not support Trump, so it is possible that those Republicans will help the Democrats to prevent Trump from ruining the country or causing major international conflicts.