The backlash and criticism was near instantaneous, with Gillibrand replying directly to Trump on Twitter. “You cannot silence me or the millions of women who have gotten off the sidelines to speak out about the unfitness and shame you have brought to the Oval Office,” she wrote.

At a news conference later on an unrelated issue, Gillibrand called Trump’s tweet “a sexist smear attempting to silence my voice.”

“I will not be silent on this issue, neither will women who stood up to the president yesterday and neither will the millions of women who have been marching since the Women’s March to stand up against policies they do not agree with,” she added.

Gillibrand once again called on Republican congressional leaders to launch investigations into the allegations made by women against Trump, saying, “It’s the right thing to do and these allegations should be investigated, they should be investigated thoroughly. That is the right thing to do and I’m urging them to do that – as should their constituents.”

Asked about her interactions with the president, Gillibrand told reporters that Trump was “just a supporter – a supporter of my first campaign.”

Several female senators also rallied around Gillibrand, including Senator Elizabeth Warren, who pointedly asked Trump on Twitter if he were trying to “bully, intimidate and slut-shame” Gillibrand.

“Do you know who you’re picking a fight with?” Warren said. “Good luck with that, @realDonaldTrump.”

Sen. Jeanne Shaheen, also weighed in on Twitter, writing that there is “nobody tougher than @SenGillibrand & she won’t be intimidated. Women will continue to speak up.”

Gillibrand was attending a bipartisan bible study on Tuesday morning when Trump’s tweet landed, and her phone was immediately filled with supportive and befuddled messages, wondering just what the president was thinking, a Gillibrand aide said.

Gretchen Carlson, the former Fox News personality whose lawsuit against Roger Ailes for sexual harassment led to the resignation of the late network chairman, also weighed in with a duo of tweets defending Gillibrand.

“What do u mean @SenGillibrand would ‘do anything’ for campaign contributions? By the way she isn’t a lightweight,” she wrote. In a second tweet, Carlson continued: “Sexual harassment is apolitical. Women will not be silenced no matter what party they are in. Period.”

Katty Kay, an anchor for BCC World News America, also took to social media to respond to the president’s missive against Gillibrand, casting it in tweets as “clearly sexual” and “demeaning to women.”

“What is so maddening about the Gillibrand tweet is that women can be smart, work hard, become Senator and STILL get sexual c**p thrown at us,” she wrote. “Enough.”

Trump offered no evidence to support his claim that Gillibrand had gone to him “begging” for campaign donations “and would do anything for them.”

According to Open Secrets, a non-profit website that tracks campaign contributions, since 1996, Trump has donated US$8,900 to Senate Minority Leader Charles Schumer, and US$5,850 to Gillibrand.

Gillibrand met with Trump once in 2010, the Gillibrand aide said, and Trump’s oldest daughter, Ivanka, attended the meeting.

On Monday, Gillibrand, a leading voice in Congress for combating sexual assault in the military, became the fifth Democratic senator to call on Trump to step down because of the allegations of sexual misconduct against him – accusations the president has denied and the White House dismissed again on Monday.

“President Trump has committed assault, according to these women, and those are very credible allegations of misconduct and criminal activity, and he should be fully investigated and he should resign,” Gillibrand said on CNN. “These allegations are credible; they are numerous. I’ve heard these women’s testimony, and many of them are heartbreaking.”

She joined Democratic Senators Cory Booker, Jeff Merkley, Ron Wyden, and Bernie Sanders in calling for Trump’s resignation.

Trump has not commented on the male senators’ demand that he resign.

Gillibrand, New York’s junior senator and a rising political star, is widely considered a likely 2020 presidential candidate against Trump, and the president’s Twitter assault on Tuesday offered an early glimpse of just how vicious the next race for the White House could become.

Gillibrand, however, does have her critics. After she said in November that former president Bill Clinton should have resigned the presidency following his inappropriate affair with White House intern Monica Lewinsky, long-time Hillary Clinton adviser and confidant Philippe Reines excoriated her on Twitter for being ungrateful and two-faced.

The White House did not respond to a request for comment about why the president sent the tweet, or what exactly he was insinuating.