First, a president’s approval rating heavily influences midterm elections. The outcome in the campaign for the House will presumably be much different depending on whether Trump is at 35 or 45 percent. Republicans dumping Trump wouldn’t make him any more popular.
Second, such a distancing is not really politically practicable. If Republicans try to skitter away from Trump, their base will roast them. There’s no reason to think that at this point the dynamic would be any different than after the release of the “Access Hollywood” tape, when Republicans dumping Trump were quickly forced to pick him right back up again.
Third, Republicans want to get some things done legislatively. A poisonous split with the White House wouldn’t help. Trump may be a mercurial and frustrating partner, but he is a partner all the same.
Finally, most Republicans — quite legitimately — think the Russian controversy is a media-driven travesty. If there were a smoking gun, this posture would probably change (obviously, in that circumstance, it should change).