President Trump is a "bully" who has shown "cruelty" toward immigrants, and thus represents a darker side of America, presidential historian Douglas Brinkley said Monday.

"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," Brinkley said on C-SPAN.

He said Trump's proposed border wall reflects that cruelty.

"The wall is just this idiotic idea. I mean, anybody goes down and travels the border with the Rio Grande, with the canyons ... I guess you're looking at ... additional border fencing in some patches down there."

"But the whole thing just became just a giant metaphor, the wall, for anti-immigration and an anti-Mexican sentiment," Brinkley said. "It's not a pretty chapter in our nation's history."

"Donald Trump represents kind of a dark underbelly of America," he added. "He's put together a coalition of people that don't like otherness. There's xenophobia that's going on here."

The Rice University professor also said Trump is not a president that children can look up too because he "lies too much" and straddles the line between morality and immorality.

"It is sad that the kids sometimes ... see we don't have someone that you can tell your kids, 'I hope you grow up to be like Donald Trump,'" he said. "I would be worried about parents that say that."

"Donald Trump lies too much, he conflates 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 asked. "That there are no rules, break them all ... dangle a foot over the line of what's moral and immoral?"

Brinkley also dismissed Trump's business background, and said calling him a businessman "kind of insults people that do business in a kind of open-handed way."

"I mean, you don't create Trump University as a businessman," he said. "That's somebody that's trying to pull a scam. That's somebody who's a carny, a kind of hypersalesman, right, the casino bellringer. 'Come on in and get rich.'"

Despite those criticisms, Brinkley said Trump could still win re-election, but also said Trump is at risk if his Twitter feed becomes stale.

"If he starts losing that as being his potent weapon, if the tweets don't carry the weight anymore because every single one is inflammatory in some way, then he I think loses even more of the power he's accumulated," he said.

In response to a C-SPAN caller, Brinkley insisted that he doesn't hate Trump, and instead said he is "disappointed" in Trump, just as he was when former President Bill Clinton got caught lying under oath about a sexual relationship he had with a White House intern.

"Be normal, and run the country," he said.