Presidential historian Douglas Brinkley on Monday tore into President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, referring to him as a "bully" and criticizing his views on immigration.

"Donald Trump represents kind of a dark underbelly of America," Brinkley said on C-SPAN.

He said Trump had put together a coalition of people that "don't like otherness."

Brinkley also addressed Trump's proposed border wall along the U.S.-Mexico border and his views on immigration.

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"He will say it's all about national homeland security, but there's been a cruelty about the way he's dealt with the immigration issue that I find unforgivable," he said.

He called the proposed border wall an "idiotic idea."

"The whole thing just became just a giant metaphor, the wall, for anti-immigration and an anti-Mexican sentiment," he said.

He also accused the president of lying too much and conflating things.

"He's a bully. He misuses cyber in a way to divide us. What is there to want your children to learn from?" he said.

"That there are no rules, break them all ... dangle a foot over the line of what's moral and immoral?"

Brinkley has in the past been critical of Trump. Earlier this year, he said no president had been so racially insensitive as Trump.