Part of the reason some people may believe stories like these is how Trump publicly denigrated McCain and the family of the dead muslim. He is a little careless with words. McCain was an idiot and a jerk and an adulterer and perhaps much worse. Going after him in a way that seemed to belittle his POW captivity was unwise.
On the other side of that, the narrative surrounding McCain did and does bear scrutiny, particularly in light of his “maverick” behavior while in office.
And it does not look like Trump’s demeanor in that regard is costing him any voters.
My husband always says that Trump comment was a huge mistake--McCain fought Trump at every turn for the rest of his life and maybe partly because of that (but maybe he would have done that anyway). Just not very wise when trying to get stuff passed in the Senate.
Still, that in no way means Trump would have called World War I American soldiers "losers."