First and foremost of their fears is Robert Mueller's criminal investigation. Two important witnesses have flipped and prosecutors are sifting through palace secrets. This undoubtedly is part of the reason Trump has been looking and acting so badly.
Second is the fact that legislatures are notoriously quarrelsome and difficult to corral in election years. Least anyone forget, in 1974, Congress started the year strongly in Richard Nixon's camp, but by mid-summer, they were itching to impeach him.
Third is Trump's looming first midterm election, a time when the party controlling the White House historically loses seats and often very badly. Only twice in American history has a sitting President gained Congressional seats in his first midterm election (details here). Given how horribly the GOP performed in November and December 2017 elections, the prognosis for Trump picking up any support in this year's election are slim.
Fourth is the strong possibility that if either the House or Senate fall out of Republican control -- or both -- it's all but a certainty that Congressional hearings would begin on a whole host of Trump's corruption and transgressions.
And fifth is the fact that Trump's White House is already experiencing a historic exodus as people rush for the exits, realizing that experience in his administration is resume poison. As often happens with stampedes, they only get worse.
All of this certainly is demoralizing Trump's people. That will cause them to be incautious and careless and thus more likely to cut their own throats.
It couldn't happen to a nicer bunch of douchebags.