WASHINGTON ― President Donald Trump on Tuesday responded to calls for his resignation by attacking Sen. Kirsten Gillibrand (D-N.Y.) in a tweet that implied she was willing to trade sex for campaign contributions.

Trump, in an early-morning tweet the day after four of his accusers called on Congress to investigate him, first blamed Democrats for the “false accusations” involving “women who I don’t know and/or have never met.”

Despite thousands of hours wasted and many millions of dollars spent, the Democrats have been unable to show any collusion with Russia - so now they are moving on to the false accusations and fabricated stories of women who I don’t know and/or have never met. FAKE NEWS! — Donald J. Trump (@realDonaldTrump) December 12, 2017

In a second tweet, Trump attacked Gillibrand as “lightweight” and “a total flunky” for Senate Minority Leader Chuck Schumer (D-N.Y.). He claimed she “begged” him for campaign donations in the past and “would do anything for them.”

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. Very disloyal to Bill & Crooked-USED! — Donald J. Trump (@realDonaldTrump) December 12, 2017

Gillibrand responded to Trump’s tweet by calling it a “sexist smear.” She also pointing out that “millions of women” protested Trump’s inauguration and have been stirred into activism since his election.

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. https://t.co/UbQZqubXZv — Kirsten Gillibrand (@SenGillibrand) December 12, 2017

Sen. Elizabeth Warren (D-Mass.) accused the president of attempting to “slut-shame” Gillibrand.

“Disgusting, it was disgusting,” she told reporters of Trump’s tweet. “It’s clear what he was getting at.”

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. https://t.co/mYJtBZfxiu — Elizabeth Warren (@SenWarren) December 12, 2017

More than a dozen women have accused Trump of sexual harassment or assault. Among them are a former business partner, contestants on his reality television show “The Apprentice,” and a People magazine reporter who interviewed him and his wife, Melania, in 2005.

Some of Trump’s accusers have revived their stories amid the wave of misconduct claims against prominent men in politics, media and entertainment, as well as Trump’s support for Alabama GOP Senate candidate Roy Moore, who faces allegations that he sexually assaulted and preyed on teenage girls when he was in his 30s.

Four of Trump’s accusers this week asked Congress to hold him accountable for his actions.

Several Democratic senators, including Gillibrand, Cory Booker (N.J.) and Jeff Merkley (Ore.), have called on Trump to quit. On Monday night, a group of more than 50 female House Democrats wrote a letter to the House Oversight Committee to demand a congressional probe, CNN reported.

The president and the White House have repeatedly claimed that all of the women who have come forward against Trump are liars, and that Trump’s electoral victory last year should have put the issue to rest.

“The people of this country, in a decisive election, supported President Trump, and we feel like these allegations have been answered through that process,” press secretary Sarah Huckabee Sanders told reporters on Monday.

Trump reportedly is also trying to cast doubt on the authenticity of the infamous 2005 “Access Hollywood” recording of him bragging about sexual assault.

On Tuesday, Sanders defended Trump’s tweets, saying it was “very obvious” that he was referring to “the corruption of the entire political system.”

“He’s not alleging anything,” Sanders said at Tuesday’s White House press briefing. “There’s no way that this is sexist at all.”

Sanders again referred to the sexual misconduct allegations as “false and fabricated,” saying that questions about them had been “asked and answered” during last year’s election.

Igor Bobic contributed reporting.

This article has been updated to include Gillibrand and Warren’s responses to Trump, and comments from White House press secretary Sarah Huckabee Sanders on Tuesday afternoon.

CORRECTION: A previous version of this story misidentified Senate Minority Leader Chuck Schumer as majority leader.