The magic of denial and wishful thinking. The GOP acts like Trump being impeached will boost him because it helped Clinton. The difference is that Clinton had about a 69% or so approval rating when he got impeached. Trump has never even broken 50% approval outside biased outliers like Rasmussen.
Trump has had an abysmal approval rating since taking office despite the relatively good economy. If it falters even a little before the election next year, he is pretty much guaranteed to be a goner.
The GOP base might love him, but the Democratic base hates him and is larger than the GOP base and independents are souring on Trump. Dems need to run ads targeting independents stating "You have had four years to see who Donald Trump really is. He will never change who he is because he thinks he is perfect. Do you really want four more years that will be identical to the last four?"