Republican presidential candidate Donald Trump speaks at a campaign event. (AP/Jim Cole)

Republican presidential candidate Donald Trump vowed to escalate his attacks on Democratic front runner Hillary Clinton Monday night, telling comedian Jimmy Fallon during an appearance on the “Tonight Show” that he has not “even started" tearing into the former secretary of state.

“I haven’t even started on her yet — although last week I did a little bit,” Trump told Fallon, foregoing his notoriously bombastic style during the interview in favor of a notably muted tone. “But we haven't even started."

The celebrity billionaire has repeatedly condemned Clinton and her husband former president Bill Clinton in recent weeks, accusing the former president of sexual harassment and suggesting the former first lady enabled him. Asked about his Democratic rival Monday, Trump indicated his attacks were merely a response to Clinton’s negative characterizations of him on the campaign trail.

"She came out with a little bit of a statement about me, and I came out with a very big statement about her and Bill,” Trump told Fallon, prompting the crowd to laugh. "And she stopped talking about me all of a sudden."

Trump and his campaign staffers have repeatedly made references to the former president’s sexual improprieties on the campaign trail in recent weeks. The former secretary of state responded crossly Sunday during an appearance on CBS’ “Face the Nation.”

"If he wants to engage in personal attacks from the past, that's his prerogative," she said. "I'm going to draw the distinctions between where I stand and where he stands.”

"He can say whatever he wants to about me. Let the voters judge that, but I am not going to let him or any of the other Republicans rip away the progress that women have made," Clinton added.

A Fox News poll released Friday showed Trump leading Clinton 47 to 44 percent in a hypothetical general election match up. A CNN/ORC poll released before Christmas showed Clinton leading Trump 49 to 47 percent.