It’s because Ted Cruz was a campaign staffer for George W. Bush and McCain holds a bitter grudge against the “Bushies” for his primary defeat in South Carolina in 2000, which ended his presidential bid that year. Lindsay Graham supported McCain in 2000 and is still angry that his candidate’s presidential bid ended in Graham’s home state.
Successful politicians have to learn to forget the inevitable insults and defeats that are bound to occur when one interacts with people in a competitive environment. Those who can’t do that will eventually have a stalled career. The exception is someone like Barack Obama who has the race card going for him and thus lives a charmed life because Black voters will support him solely on the basis of his race, and whites (including opposing candidates) are afraid to criticize him for fear of being called racists. Obama can and does hold grudges and carries a giant chip on his shoulder, and gets away with it. Most others can’t.
McCain’s vindictiveness may stem from a continuing need to take revenge on the North Vietnamese who tortured him. He can’t do that, of course. But he can engage in the defense mechanism of transference. He transfers his rage at his former captors who tortured him then to everyone who displeases him now. He holds grudges as a way of finding a compartment in his psyche for his ongoing and relentless anger over what happened to him in the Hanoi Hilton.
This is a heavy bit of baggage to carry around. Nobody can interact with others without being displeased on a routine basis and occasionally insulted. This has to be especially true in politics. All that resentment builds up until it eventually impairs cognitive function and destroys happiness.
McCain passed that milestone several years ago.