Trumps biggest liability is his thin skin. He would be served better IMO to let negative comments roll off of him. But he is a businessman that has entered the political arena. An arena he understands from the outside, not the inside.
There is some background behind Trump's comments.
http://www.newyorker.com/news/news-desk/john-mccain-has-a-few-things-to-say-about-donald-trump
McCain is just pissed because his apple cart got turned over. IMO it needed to get turned over. McCain is a squish and it's past time for him to enjoy the rest of his days pursuing interests other that the Senate.
John McCain has not functioned as a Republican for quite some time now. He needs to retire. People like Trump are moving the Republican Party back to the right. I applaud it.
It's way past time that voters had actual choices instead of progressive and progressive lite. But if Trump keeps this up, with this type of rhetoric, he will make a mockery of himself and he will have no one to blame but himself.
Whatever.