WASHINGTON (Reuters) - U.S. Senator Kirsten Gillibrand fired back at President Donald Trump on Tuesday and said she would not be silenced after he attacked her on Twitter for calling for an investigation into accusations of sexual harassment and misconduct against him.

Six U.S. senators, including Gillibrand, have said Trump should resign.

Trump lambasted Gillibrand on Twitter on Tuesday writing, “Lightweight Senator Kirsten Gillibrand, a total flunky for Chuck Schumer and someone who would come to my office ‘begging’ for campaign contributions not so long ago (and would do anything for them), is now in the ring fighting against Trump.” Schumer is the Senate Democratic leader.

Gillibrand, whose name has been floated as a possible Democratic presidential candidate in 2020, said she would not back down.

“It was a sexist smear attempting to silence my voice, and I will not be silenced on this issue,” she told reporters at a news conference.

Trump did not answer a reporter’s question at a White House event later on Tuesday when asked what he meant by the tweet.

White House spokeswoman Sarah Sanders, told that some people thought Trump’s tweet contained sexual innuendo, said, “Only if your mind is in the gutter would you have read it that way ... it’s obviously talking about political partisan games that people often play and the broken system.”

Sanders told a regular White House briefing that Trump had used similar language previously to refer to men of both major parties.

Other Democratic lawmakers rallied behind Gillibrand, including U.S. Senator Elizabeth Warren, another possible 2020 presidential candidate.

In a tweet directed at Trump, Warren wrote on Tuesday, “Are you really trying to bully, intimidate and slut-shame @SenGillibrand? Do you know who you’re picking a fight with? Good luck with that, @realDonaldTrump. Nevertheless, #shepersisted.”

U.S. Senate Democratic Leader Chuck Schumer said Trump’s attack on Gillibrand was “nasty, unbecoming of a president,” but he did not join her call for Trump to resign the presidency over sexual misconduct accusations.

More than a dozen women have accused Trump, a New York-based real estate developer and former reality television star, of making unwanted sexual advances against them years before he entered politics. Trump, a Republican, has denied the allegations.

Reuters has not independently verified the accusations against Trump.

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Interest in accusations of sexual harassment and misconduct came to the fore again on Monday when three women who had previously accused Trump of misconduct called on the U.S. Congress to investigate his behavior.

On Tuesday, a fourth woman who had also previously made similar accusations backed their call for an investigation during an interview with NBC.

Nearly 60 female Democratic U.S. lawmakers called for an investigation in a letter on Monday.

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By Tuesday, the group said many male colleagues had also joined on, bringing the number to more than 100 lawmakers in the U.S. House of Representatives.

Representative Trey Gowdy, Republican chairman of the House Oversight Committee, responded to the group in a letter on Tuesday that said, “The specific allegations set forth in your letter constitute crimes,” both federal and state.

Gowdy noted that congressional panels cannot prosecute crimes so he was forwarding the group’s letter to the Justice Department. He added that any charges not alleging crimes should go to the House Judiciary Committee, which has jurisdiction over “allegations related to fitness for office and non-criminal matters.”

Trump has called the accusations fabricated stories and he has said he did not know his accusers.

On Monday, Gillibrand called the allegations credible and called on Trump to resign over them.

The attention to sexual harassment accusations against Trump comes amid a wave of similar accusations against prominent men in Hollywood, the media and politics in recent months.

Federal Election Commission records showed Trump gave $4,800 to Gillibrand’s Senate campaign in 2010, and that he donated $2,100 to her in 2007 while she was a member of the House of Representatives.

Concerns over sexual impropriety have become a political issue the United States, leading to the resignations of two Democratic and one Republican lawmaker. Reuters has not independently verified accusations against them.

The issue of sexual harassment has also become central to Tuesday’s U.S. Senate election in Alabama after accusations of misconduct were made against Republican candidate Roy Moore.

The White House said on Monday that the women’s accusations against Trump were false and “totally disputed in most cases by eyewitness accounts” and later promised to provide a list of those accounts to reporters.

On Tuesday, the White house sent a list of three 2016 media reports, including a New York Post interview with a British man who disputed one of the accusers’ accounts of alleged groping and said he never saw it happen. It also included New York Daily News and CNN reports with two other former pageant participants supporting Trump.