A couple of weeks ago, looking for info on John MaCain's boat trip at lake Como, I came across a piece/letter by him ( can't remember which or where ). It was calling for bi-partisan working across the floor, upholding the Constitution and made absolutely clear that he respected the office of President, but he didn't respect the current holder who shouldn't think himself better or greater for being potus....
I think Trump is worried that impeachment might be started by MaCain, especially after him being so nice about war vets and POWs.