Washington (CNN) Donald Trump on Saturday night slammed Hillary Clinton by citing her husband's history of marital infidelity and alleged sexual misconduct, escalating the increasingly personal feud between the GOP front-runner and the leading Democratic presidential candidate.

"Hillary Clinton has announced that she is letting her husband out to campaign but HE'S DEMONSTRATED A PENCHANT FOR SEXISM, so inappropriate!" Trump tweeted.

Hillary Clinton has announced that she is letting her husband out to campaign but HE'S DEMONSTRATED A PENCHANT FOR SEXISM, so inappropriate!

The "penchant for sexism" line refers to an interview Hillary Clinton gave earlier in the week to the Des Moines Register , when she used those words to describe the real estate mogul after he said she got "schlonged" in her 2008 primary loss to then-Sen. Barack Obama, a remark widely seen as sexist.

Trump then appeared to threaten Clinton over the remark in a tweet the next day.

"Hillary, when you complain about 'a penchant for sexism,' who are you referring to. I have great respect for women. BE CAREFUL!" he said Wednesday night.

"Hillary Clinton has some nerve to talk about the war on women and the bigotry toward women when she has a serious problem in her husband," Pierson told CNN's Kate Bolduan on "Erin Burnett OutFront."

Pierson went on to compare the Clinton campaign to "9-year-old little girls" and accused Clinton of bullying.

"I can think of quite a few women who have been bullied by Hillary Clinton to hide her husband's misogynist, sexist secrets," Pierson said.

Trump and Bill Clinton have a long relationship. The Clintons attended Trump's third wedding in 2005, and the businessman is a donor to the Clinton Foundation. In August, The Washington Post reported that Clinton called Trump before the brash billionaire announced his candidacy for the Republican nomination, encouraging him to "play a larger role in the Republican Party."

So far, Bill Clinton has largely avoided publicly campaigning for his wife or getting involved in the sniping between 2016 hopefuls, though he did call Trump "fact-free" during an interview with CNN's Erin Burnett in September.

Back then, he also said Trump's criticisms of Hillary Clinton would be ripped apart in the context of a general election.

"You know, if he becomes the nominee, he'll have to sort of hone his criticisms a little more finely because the facts will be easy to marshal," Bill Clinton said. "But you know, he's good at this, that's what he does. And the people that he is telling it to now basically have only heard that story, so they believe it and it's probably good politics for him."