IMHO McCain has lost credibility with those who voted for Trump.
Fact, during the debates Trump should have never made his personal attack on McCains service records. It was unnecessarily cruel and uncalled for. Even though I'm a big time supporter of Trump, I'm not blinded when he does something wrong.
Back to McCain and his lack of credibility, most conservatives know that McCain is mad as hell at President Trump and is going against the grain and against what his voters voted for to stick it to Trump. The problem is that to conservatives, McCain played the wrong cards. They feel that McCain could've played the forgive and forget card and move on, I'm thinking about the same cards that Ted Cruz played, people would've been sympathetic toward him, plus he could've played the victim.
Instead he turned bitter and angry and he doesn't care about his constituents or what they wanted him to vote for. Kasich is in the same boat as McCain, however he's polished up his act and IMHO he doesn't come off as being as hateful as McCain does.
From the movie Bareback Mountain, "Lindsey, I wish I knew how to quit you."
Graham to McCain, "Tell you what... the truth is... sometimes I miss you so much I can hardly stand it."