WASHINGTON — Senator Al Franken of Minnesota, in an emotional speech on the Senate floor, announced on Thursday that he would resign from Congress, the most prominent figure in a growing list of lawmakers felled by charges of sexual harassment or indiscretions.
At turns defiant and mournful but hardly contrite, Mr. Franken called it “the worst day of my political life,” as he denied allegations of groping and improper advances from at least six women. Instead, as his Democratic colleagues looked on, he took a parting shot at President Trump and Roy S. Moore, the Republican candidate for Senate in Alabama; both have also been accused of sexual misconduct.
“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 said.
Hours later, Representative Trent Franks, Republican of Arizona, resigned after the House Ethics Committee opened an investigation into allegations of sexual harassment — a move that made him the third member of Congress to leave under a cloud of claims of sexual impropriety in three days. On Tuesday, Representative John Conyers Jr., Democrat of Michigan and the longest-serving African-American in House history, also quit.
The dizzying series of departures comes during a national reckoning over sexual misconduct in the workplace that has cost men their jobs across the spectrum of American life, including in the entertainment and media industries, the arts, academia and now, politics. As accusations of sexual improprieties continue to swirl on Capitol Hill, lawmakers have struggled with how to respond.
Democrats and their leaders forced Mr. Conyers and Mr. Franken out in a succession of coordinated statements that made clear that their continued presence would be untenable. Mr. Franken stepped down one day after nearly all the Senate’s Democratic women — and most Democratic men, including the top two leaders — called for him to resign.
Democrats appear determined to grab the moral high ground in an environment in which they hope sexual harassment becomes a wedge issue in the 2018 midterm elections — even if it costs them popular colleagues and political icons.
Republicans, by contrast, have been more situational. In the case of Mr. Franks, Speaker Paul D. Ryan of Wisconsin released a statement Thursday night indicating that he had forced the representative out.
“The speaker takes seriously his obligation to ensure a safe workplace in the House,” a statement from Mr. Ryan’s office said.
Yet Representative Blake Farenthold of Texas appears to be under little pressure, even though he used $84,000 in taxpayer funds to settle a sexual harassment claim with his former communications director. The House Ethics Committee said on Thursday that it was establishing a subcommittee to investigate Mr. Farenthold.
Mr. Moore’s Senate candidacy in Alabama has surfaced accusations that he sexually molested or assaulted girls as young as 14, yet he continues to have the support of Mr. Trump and the Republican National Committee. And allegations of sexual assault and harassment against Mr. Trump have hardly shaken his control over the party.
“The Democrats are making a smart political calculation,” said Peter Wehner, who advised former President George W. Bush on domestic policy. “I think they probably saw the political opportunity, and they couldn’t take advantage of it unless they jettisoned their own problematic figures.”
Mr. Wehner continued: “After years of making the argument that character mattered in terms of sexual ethics, now Republicans are saying it doesn’t matter at all. They’re utterly indifferent to it, and Republicans and evangelical Christians have nothing to do with this particular moral moment.”
But Democrats also face risks in setting themselves up as the party of purity. Some members of the Congressional Black Caucus are angry over the treatment of Mr. Conyers, who served in the House since 1965. Some progressive Democrats, who form Mr. Franken’s base, view the treatment of him as an overreaction.
“The moment he was targeted they decided to eat their own, and we do this to ourselves all the time,” said Natalie Volin Lehr, a former aide to Mr. Franken who oversaw women’s outreach for him. “I think there’s a sense of hyper political correctness and that we’re holier than thou.”
Mr. Franken’s announcement on Thursday was a jarring end to an improbable political career in which the senator, a founding writer and performer on “Saturday Night Live,” narrowly won a seat in 2008 and offered Democrats a crucial vote needed to advance the Obama administration’s agenda, including the Affordable Care Act.
Mr. Franken cut a serious figure in the Senate, where he sought to stifle his sense of humor as he dove into meaty policy issues like electronic privacy and telecommunications mergers. Some viewed him as a potential contender for the presidency in 2020.
The accusations against him began last month when Leeann Tweeden, a radio news anchor in California, accused Mr. Franken of forcibly kissing and groping her on a U.S.O. tour in 2006. Several women also said Mr. Franken groped them as he posed with them for photographs, mostly before he became a senator.
Over the last three weeks, Mr. Franken has repeatedly apologized for his behavior, although he has also challenged some of the accusations of impropriety. Until Wednesday, he had said he would remain in his job, but his Democratic colleagues in the Senate made clear this week that his apologies and admissions were not sufficient.
Senator Tim Kaine, Democrat of Virginia, said that Mr. Franken’s resignation crossed “a cultural Rubicon” and set a new standard by which future senators would be judged.
“Now it’s clear that behavior of this kind before you take office is something that the body should take seriously — and it should be evenhanded whether you are a Democrat or a Republican,’’ Mr. Kaine said.
The Senate was somber as Mr. Franken delivered his speech on Thursday. His staff and family, including his wife, Franni Bryson, watched from the gallery above. About 20 Democrats and, apart from Senator Dan Sullivan, Republican of Alaska, who was presiding, just one Republican — Senator Jeff Flake of Arizona, an ardent critic of Mr. Trump — arrived in the chamber to hear his remarks.
When Mr. Franken was through, many of the same lawmakers who had called on him to resign lined up, one by one, to hug him. Senator Claire McCaskill, Democrat of Missouri, wiped away tears.
In his speech, Mr. Franken, who counted himself an ardent defender of women’s rights, called the national reckoning “an important moment” that was “long overdue,” adding, “We were finally beginning to listen to women about the ways in which men’s actions affect them.” He said he was “excited for that conversation” and hoped to be a part of it.
“Then,” he said, “the conversation turned to me.”
Mr. Franken said he decided to leave office because it became clear that he could not both pursue an Ethics Committee investigation and represent the people of Minnesota. He maintained that he would have ultimately been cleared.
“Some of the allegations against me are simply not true,” Mr. Franken said. “Others I remember very differently.”
“I know in my heart, nothing that I have done as a senator, nothing, has brought dishonor on this institution,” he said.
Mr. Franken did not specify precisely when he would leave the Senate, saying only that he would do so “in the coming weeks.”
It will be up to the Democratic governor of Minnesota, Mark Dayton, to choose a successor for Mr. Franken who will serve until November 2018. Mr. Dayton is expected to choose from among several prominent Democratic women, including Lt. Gov. Tina Smith and Attorney General Lori Swanson. The governor said on Thursday that he was still weighing his decision, but his choice was certain to upend the state’s politics.
Over time, though, Mr. Franken’s resignation could become a headache for Democrats as the former governor of Minnesota, Tim Pawlenty, a Republican, is being heavily recruited to enter the special election race next year for the Senate seat by colleagues in the state and in Washington.
In the Senate, Mr. Franken’s influence was more political than legislative. He was popular on the party’s fund-raising circuit and among his colleagues, but he sought to show he could do more than tell jokes. Since Mr. Trump’s ascent to office, Mr. Franken’s sharp questioning of the president’s nominees and policies became must-see TV in the capital.
His questioning of Jeff Sessions, then a senator, during his confirmation hearing to be attorney general in January may prove to be one of his most important contributions as a senator. He pressed Mr. Sessions about reports of contacts between the Trump campaign and Russian officials, to which Mr. Sessions replied: “Senator Franken, I’m not aware of any of those activities. I have been called a surrogate at a time or two in that campaign and I didn’t have — did not have — communications with the Russians.”
The exchange helped force Mr. Sessions to recuse himself from the Justice Department’s investigation into Russia’s interference in the presidential election and helped prompt the naming of a special counsel, Robert S. Mueller III, who has dogged the Trump administration.
Senate Democrats were reluctant on Thursday to fully address the arc of Mr. Franken’s downfall, blazing past reporters or offering only brief thanks that Mr. Franken put an end to the uncertainty.
Leaving the Capitol shortly after his speech, Mr. Franken said he would not be taking questions.
But asked whether he had a message for his home state, he said, “I’ll be coming home.”