I've heard it said that if the GOP could overturn Obamacare quickly then it wouldn't be on the voters' minds come 2018, and any fallout from that they could find a way to explain away.
But if Trump continues to be Trump (and he shows no signs of stopping), and the GOP continues to carry his water, there will still be a lot of anger and fear in voters' minds come the midterm elections. The longer Republicans keep this up the more they hitch their individual fates to his.
But from what I've seen of the districts being contested, it will be difficult for the Democrats to win enough seats to mount a serious impeachment bid alone (2/3 majority in the Senate needed to convict.) Watergate is the distant past, and since the "Reagan revolution" the GOP has closed ranks; they aren't really prepared to go after one of their own. In 2020 the GOP will have a lot more seats to defend in the Senate, and that could mean a complete change in power for Congress and the White House. And the longer Trump stays in the White House the more long term damage he does to the GOP. I guess that's the silver lining on this pitch black storm cloud.