Democratic Senator and ex-comedian Al Franken has said he plans to quit "in the coming weeks" after string of sexual harassment allegations.
"I am proud that during my time in the Senate that I have used my power to be a champion of women," the Minnesota senator said from the US Senate floor.
His speech came a day after nearly 30 Democrats called on him to resign.
He would be the most prominent lawmaker to resign amid a wave of misconduct claims against high-profile figures.
Meanwhile, the US House of Representatives Ethics Committee launched sexual harassment investigations into two Republican congressmen.
Trent Franks of Arizona announced he was resigning as the inquiry was announced.
He acknowledged having made two female congressional aides "uncomfortable" by asking them about surrogacy when he and his wife faced infertility.
The committee also said it would investigate Blake Farenthold, who used $84,000 (£62,000) of taxpayers' money to settle a sexual harassment lawsuit with his former spokeswoman.
Over in the Senate, Mr Franken told his colleagues on Thursday: "Today I am announcing that in the coming weeks I will be resigning as a member of the United States Senate.
"I may be resigning my seat but I am not giving up my voice."
The former Saturday Night Live comic and two-term senator has apologised to several women who have accused him of groping and sexual harassment, but he faced mounting pressure to step aside after a new allegation surfaced on Wednesday.
Mr Franken said some of the claims against him "are simply are not true", but added that women "deserve to be heard and their experiences taken seriously".
He also referenced the sexual misconduct allegations that have been levelled against President Donald Trump and Republican Alabama Senate candidate Roy Moore.
"I, of all people, am aware that there is some irony in the fact that I am leaving while a man who has bragged on tape about his history of sexual assault sits in the Oval Office, and a man who has repeatedly preyed on young girls campaigns for the Senate with the full support of his party."
Mr Franken is not the only US politician to have found himself engulfed by sexual harassment in recent weeks.
On Tuesday, Michigan Democrat John Conyers announced he would resign amid claims of sexual harassment made by his congressional aides.
Seven women have come forward to accuse Mr Moore, a former Alabama Supreme Court judge, of sexual misconduct decades ago.
Several Democratic female senators - including some who called for Mr Franken's resignation a day earlier - hugged the lawmaker after his speech.
Senator Bernie Sanders echoed Mr Franken's criticisms of Mr Trump on Twitter.
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Fellow Minnesota Senator Amy Klobuchar thanked Mr Franken on Facebook, calling him a "friend to me and many in our state".
"Nothing is easy or pleasant about this," she wrote, "but we all must recognise that our workplace cultures - and the way we treat each other as human beings - must change."
The decision to fill the vacancy left by Mr Franken will fall to Democratic Minnesota Governor Mark Dayton, who said in a statement he has not determined who will replace him.
"I extend my deepest regrets to the women who have had to endure their unwanted experiences with Senator Franken," he said.