Mr. Trump said Monday night, “She was favoured to win and she got schlonged, she lost,” using a Yiddish slang word for male genitalia.

‘It is not vulgar’

The Republican frontrunner argued with that definition on Twitter Tuesday. “’Schlonged’ is not vulgar,” he wrote. “When I said Hillary got ‘schlonged’ that meant beaten badly.”

Mr. Trump, who has ramped up his criticism of Ms. Clinton in recent weeks, also mocked the Democratic frontrunner on Monday for returning late to Saturday’s debate following a commercial break because she’d been using the bathroom.

‘Disgusting where she went’

“What happened to her? I’m watching the debate, and she disappeared. Where did she go?!” Mr. Trump said at a rally in Michigan.

“I know where she went. It’s disgusting, I don’t want to talk about it,” he added. “No, it’s too disgusting. Don’t say it, it’s disgusting.”

Incendiary rhetoric

Mr. Trump continues to the lead the crowded field of candidates vying for the Republican nomination in primary contests that start in just over six weeks.

The development has alarmed many Republican Party leaders, who fear the billionaire’s incendiary rhetoric would make him unelectable in a general election face-off with Ms. Clinton, who is widely favoured over two rivals for the Democratic nomination.

Not the first time

This isn’t the first time Mr. Trump has used what could be interpreted as gendered language to criticise Clinton. He has repeatedly described her as lacking the strength and stamina to be President and invoked the image of her putting on a pantsuit before heading to work.

In an interview on Tuesday with The Des Moines Register, Ms. Clinton said: “Nothing really surprises me anymore. I don’t know that he has any boundaries at all. His bigotry, his bluster, his bullying have become his campaign. And he has to keep sort of upping the stakes and going even further.”

Latest rift after video remark

The latest rift between Ms. Clinton and Mr. Trump began during Saturday evening’s Democratic presidential debate. Ms. Clinton claimed the Islamic State group had used videos of Mr. Trump’s comments about Muslims to recruit militants.

“They are going to people showing videos of Donald Trump insulting Islam and Muslims in order to recruit more radical jihadists,” she said.

There was no evidence to back the claim, and Mr. Trump quickly called Ms. Clinton a liar and demanded an apology. “She’s a liar!” he repeated at Monday’s rally, labelling Ms. Clinton “not a President.”

Ms. Clinton appeared on Tuesday in Keota, Iowa, a small rural town where three high school students launched an energetic campaign to get her to visit their school.

‘Trumps’ anti-Muslim rhetoric

Ms. Clinton decried Mr. Trump’s rhetoric on Muslims, saying it was “not only dangerous, it’s shameful.” She said that kind of talk played in to the hands of violent jihadists.

“It’s a very serious matter when you hear some of the things he and unfortunately some of the others have been saying,” Ms. Clinton said.

Clinton did not reference Trump’s latest comments on her 2008 presidential bid, but she made a veiled reference to his campaign when a female student asked her about combating bullying.

Stand up to bullies