US President Donald Trump has been accused of trying to "slut shame" a female senator who demanded he quit over sexual misconduct claims.
Mr Trump claimed Kirsten Gillibrand had come "begging" to him for donations and "would do anything" for cash.
Senator Elizabeth Warren said Mr Trump was "trying to bully, intimidate and slut-shame" her fellow Democrat.
The White House dismissed claims that the remarks were sexist, adding that he was referring to political corruption.
"There's no way that this is sexist at all. This is simply talking about a system that we have which is broken", White House spokeswoman Sarah Sanders said on Tuesday.
Mrs Sanders said the comments had "the same sentiment that the president has expressed many times before when he has exposed the corruption of the entire political system".
She added that he has used "similar terminology many times" to discuss both men and women, adding that "politicians repeatedly beg for money".
Scores of Democratic congresswomen are urging Congress to investigate claims against the Republican president.
Three of his accusers held a press conference on Monday to repeat their allegations he groped, fondled, forcibly kissed and harassed 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
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
The Republican president has branded the accusations "fabricated" and "FAKE NEWS!"
In Tuesday morning's tweet, the US president accused Mrs Gillibrand of being a lackey to Democratic Senate leader Chuck Schumer.
"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," the US president posted.
Mr Trump did not explain what he meant by "do anything" for campaign contributions.
He added that the New York senator, who took over Hillary Clinton's Senate seat when her term ended, had been "very disloyal" to the Clintons and had been "USED" by them.
Last month, Mrs Gillibrand said former President Bill Clinton should have quit over his affair with a White House intern. A top Hillary Clinton aide called Mrs Gillibrand a "hypocrite" for that attack.
Responding to Mr Trump's tweet on Tuesday, Mrs Gillibrand wrote: "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."
.@realDonaldTrump is a misogynist, compulsive liar, and admitted sexual predator. Attacks on Kirsten are the latest example that no one is safe from this bully. He must resign. https://t.co/7lNI23K7ib— Senator Mazie Hirono (@maziehirono) December 12, 2017
Later she held a press conference to accuse Mr Trump of using a "sexist smear".
"I will not be silenced on this issue," she said, adding that "neither will the women who stood up to the president yesterday".
Democratic Senator Richard Blumenthal, a frequent critic of the president, tweeted that "America must reject Trump's sexist slurs".
Also on Tuesday, a group of 59 female members of Congress urged the House Committee on Oversight and Government Reform to investigate misconduct claims by "at least 17 women" against Mr Trump.
"The American people deserve a full inquiry into the truth of these allegations," said the letter to the committee from the Democratic Women's Working Group (DWWG).