Donald Trump slammed Massachusetts Sen. Elizabeth Warren on Monday for being “racist.”

“She made up her heritage, which I think is racist. I think she’s a racist, actually, because what she did was very racist,” Trump told NBC.

“We call her ‘pocahontas’ for a reason,” Trump said, in reference to what he’s called her on Twitter and during campaign events.

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Trump’s attacks came after Warren called the billionaire businessman a bigot and referred to his trademark baseball hats as “goofy” at a campaign event with Hillary Clinton in Cincinnati earlier that day.

“She said she’s 5 percent Native American. She was unable to prove it. She used the fact that she was Native American to advance her career. Elizabeth Warren is a total fraud. I know it. Other people who work with her know it,” Trump said.

Warren is being viewed by many political experts as a viable vice-presidential running mate for Clinton and Trump welcomed the prospect, saying he would “speak very openly about her” should she enter the race — something he’s already been doing for months.

“She’s got about as much Indian blood as I have,” Trump told a New York Times columnist in March. “Her whole life was based on a fraud. She got into Harvard and all that because she said she was a minority.”

To read about the likelihood of Warren and Clinton as running mates, click here.