He is a funny swampy guy, but he is also master of the senate and senate procedure. I don't know the details as to exaclty why, but after an apparently rocky start he's done everything he can to advance the president's agenda.  Two years ago I'd have him pegged as a potential Never Trumper Establishment member.  Also I am looking more and more at just how prescient he was in holding open the Scalia seat which Trump was then able to fill. As swampy as he appears he seems to be a fairly principled guy when you look at his actions.