Mr. Trump said that Mrs. Clinton, who has portrayed Ms. Machado as a victim of Mr. Trump’s cruel insults, had “made this young lady into a girl scout when she was the exact opposite.” He asserted, without offering any evidence, that Ms. Machado had once participated in a sex tape.

Mr. Trump, the Republican presidential nominee, argued that Mrs. Clinton’s support for Ms. Machado was part of a pattern by the Democrat of treating women to suit her own political ends, and raised Mrs. Clinton’s criticism of women who had been involved with her husband, such as Monica Lewinsky and Gennifer Flowers.

He said he was bringing up Mr. Clinton’s infidelities because he thought they would repulse female voters and turn them away from the Clintons, and because he was eager to unsettle Mrs. Clinton in their next two debates and on the campaign trail.

“She’s nasty, but I can be nastier than she ever can be,” Mr. Trump said.

“Hillary Clinton was married to the single greatest abuser of women in the history of politics,” he added about Mr. Clinton. “Hillary was an enabler, and she attacked the women who Bill Clinton mistreated afterward. I think it’s a serious problem for them, and it’s something that I’m considering talking about more in the near future.”