Donald Trump, left, speaks with New Jersey Gov. Chris Christie, right, as he attends a ceremony at the Sept. 11 memorial in New York on Sunday. (Andrew Harnik/Associated Press)

Donald Trump said Monday that Hillary Clinton has committed the "single biggest mistake of the political season" by saying at a fundraiser on Friday night that half of Trump's supporters fall into a "basket of deplorables." Trump said the remark is "much worse" than when Mitt Romney said at a 2012 fundraiser that 47 percent of Americans are dependent on the government.

"Personally, when I heard it I thought that it was not something that was within the realm of possible, that she would have said it. And I said to my people: 'I don't believe that she said it. I think you have to check it, because there's no way that she said this,' " Trump said Monday morning on Fox News Channel. "And she actually did, and she even really doubled up, because it was said with such anger, such unbelievable anger. And I think this is the biggest mistake of the political season. I really do. When I saw this in its full form, and I saw the anger with which she said it -- I mean, the way she spoke -- I think it's the single biggest mistake of the political season."

Clinton has since expressed regret for using the word "half," but Trump said Monday that she has alienated some voters.

"Remember this: If you're going to be president, you're president of all the people," he said. "You're not president of 50 percent or 75 percent. You're president of all the people. You're president of everybody."

Trump's campaign on Monday released a television ad that focuses on Clinton's "deplorables" comment and will soon air in the battleground states of Ohio, Pennsylvania, North Carolina and Florida. The ad opens with video footage from the Friday night fundraiser where Clinton said that "you could put half of Trump's supporters into what I call the 'basket of deplorables'... the racist, sexist, homophobic, xenophobic, Islamophobic, you name it."

The ad then shows crowds at Trump's rallies as the narrator says: "People like you... you... and you. You know what's deplorable? Hillary Clinton viciously demonizing hardworking people like you."