(CNN) On July 18, 2015, then-candidate Donald Trump said this about John McCain: "He's not a war hero. He was a war hero because he was captured. I like people who weren't captured."

On March 19, 2019, President Donald Trump offered this assessment of the late war hero and senator: "I was never a fan of John McCain and I never will be."

Those two comments -- almost four years apart and more than six months after McCain's death -- provide telling bookends to understand just how much Trump has changed Republican politics (and politics generally), and not for the better.

When Trump initially attacked McCain as something less than a war hero in 2015, it was covered as the end of a campaign that never really got started. Trump has been in the race for all of a month. He was still an asterisk in most polling. And everyone who knew anything assumed that attacking McCain's five years spent as a prisoner of war in Vietnam -- a time that left the Arizona Republican with lifelong wounds -- was a death sentence of Trump's political ambitions.

After all, while plenty of Republicans didn't agree with McCain's much-touted renegade nature -- and his willingness to buck party leadership -- no one ever questioned the man's service to the country (in the military and in elected office). And doing so was seen as the easiest way to destroy your political future.

Read More