Do you have any substantive reasons why you think that is?
Maybe it's just me being cynical, but many of the people (on the GOP side) that seemed to hate McCain all of the sudden, seemed to be because of Trump making him out to be some sort of "bad guy" when they had supported McCain just a few years prior.
McCain got the same treatment from Trump as guys like Jeff Sessions. Life long conservatives having it implied that they're "not really conservative" or a "sellout" by a guy who only joined the republican party in 2012, for the cardinal sin of disagreeing with him 2 or 3 times. The same reason people in the GOP have turned on Romney despite ardently defending him 10 years ago.
But I'll give you the benefit of the doubt...do you have any long-held objections to McCain's character or reasons for thinking he's corrupt that pre-date 2016?