They have already done that as many times to Trump as have been done to all other Presidents in US history combined; without conviction and removal, which requires a 2/3 supermajority in the Senate, impeachment has proven to be an ineffective constraint on Trump.
> If the Democrats take the House they can impeach him.
So? They've literally done that twice before, and what did it accomplish? They fired that bullet and missed, trying to jam it back in the gun to take another shot isn't going to work.