Democrats oppose President Trump and his policies for all sorts of reasons, but do they cross a line when they accuse him of being mentally unfit to be President?

The Goldwater Rule comes into play here; it's a reference to a magazine piece in the 1960s suggesting that psychiatrists had concerns with Arizona Senator Barry Goldwater's mental fitness to service.

In the 70s, the rule was written into the American Psychiatric Association's Principles of Media Ethics, and it prohibits psychiatrists from publicly commenting on someone's mental state who they haven't treated. That includes the President of the United States.

The rule has been increasingly discussed as of late, as more of President Trump's critics are more willing to to there.

Next asked the Tom Perez, the chair of the Democratic Party who visited Denver on Tuesday, whether that's fair game, especially in light of what the then-Labor Secretary told Kyle Clark in his appearance on Next during the campaign:

"The Commander in Chief Dr. Jekyll and Mr. Hyde, and Donald Trump is, frankly, temperamentally unfit to be the President of the United States."

He said "temperamentally" unfit, not mentally unfit.

Next's Marshall Zelinger directly asked Perez on Tuesday if he's comfortable with Democrats suggesting the President has a mental problem:

"Well listen, I'm not a doctor," Perez said. "I can't make diagnoses. And what I will say is this - this president has embarked from Day 1 on a divisive strategy of lying with regularity. I mean, this is the guy, remember, who said that Barack Obama was born in Kenya."

Perez didn't directly answer the question.

You can see the full interview here: