Former Democratic presidential candidate Martin O'Malley on Sunday called presumptive GOP presidential nominee Donald 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 a racist and a bigot who appeals to the worst instincts in people.

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"Donald Trump is a bigot. Donald Trump is a racist. Donald Trump is in fact making fascist appeals," the former Maryland governor said in a heated panel appearance on CNN's "State of the Union."

Since endorsing Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE earlier this month, O'Malley has campaigned for the presumptive Democratic presidential nominee while slamming Trump as unfit for the presidency.

"Donald Trump is an unstable charlatan who is appealing to the worst instincts in people, and I believe ultimately, the American people are going to reject that," O'Malley, who ended his own campaign in February, said on Sunday.

"That's why many self respecting Republicans are not supporting Donald Trump for president."

Panel member former Arizona Gov. Jan Brewer shot back, saying "every time you disagree with [Democrats] you're a bigot or you're a racist."

"Dang it, I get fed up that we hear over and over and over again from the president of the United States that every time someone wants to support the Constitution and the rule of law that we are out there because we are racists and bigots."

Former Virgnia Attorney General Ken Cuccinelli agreed, saying that's what happens when a president can't defend his or her policies and their outcomes.

"He jumps immediately to the race card, the racist card, in particular," Cuccinelli said.

"It's a common tactic for Democrats," he said, adding that he plans to vote for Trump in November.