In 1964, Fact magazine published an article whose headline was "1,189 Psychiatrists Say Goldwater Is Psychologically Unfit To Be President." It included a variety of detailed and unflattering diagnoses of the Republican candidate for president by psychiatrists none of whom had actually examined him or, so far as one could tell, met him.
In 2010, Christine O'Donnell, a Republican candidate for the Senate, was widely mocked as the "masturbation hating candidate." So far as I could discover, the basis for that was a comment she had made in an MTV program on masturbation some fourteen years earlier:
"The Bible says that lust in your heart is committing adultery. So you can't masturbate without lust."
Both, I think, correct statements.
That same year, another Republican senate candidate was reported as saying that he opposed the principle of separation of church and state. What he actually said was "The idea that church and state should be separated is fine with me. The idea that there should be no interrelationship between the two is not fine with me."
Those are particular incidents that struck me when they occurred–the two links above are to blog posts I made at the time. But the pattern is a general one. Center left writers and media routinely accuse candidates on the right of being ignorant, stupid, racist, and/or crazy. Most of the time it isn't true.
Donald Trump is, in my view, less qualified to be president than any major party candidate in my lifetime. But after being told more or less the same thing about every candidate seen as right of center for the last fifty years, why should voters, especially voters right of center, believe it?