That calculus may be playing in Democrats’ minds. After Politico published accusations from a former congressional staff member on Wednesday morning that Mr. Franken had forcibly kissed her, Democrats lunged. Unlike earlier accusations, the newest one involved a congressional employee in the workplace.

Mr. Schumer quietly worked through the day to urge Mr. Franken to step aside. According to a person familiar with the conversations, Mr. Schumer called Mr. Franken before any of his fellow senators went public with their resignation calls and later met with Mr. Franken and his wife in Mr. Schumer’s Washington apartment.

But that did not head off a deluge that started when Senator Kirsten Gillibrand of New York declared on Facebook, “Enough is enough.”

“As elected officials, we should be held to the highest standards—not the lowest,” Ms. Gillibrand wrote on Twitter. “The allegations against Sen. Franken describe behavior that cannot be tolerated. While he’s entitled to an Ethics Committee hearing, I believe he should step aside to let someone else serve.”

Ms. Gillibrand was joined by Senators Mazie K. Hirono of Hawaii, Claire McCaskill of Missouri, Dianne Feinstein of California, Patty Murray of Washington State, Kamala Harris of California, Tammy Baldwin of Wisconsin, Debbie Stabenow of Michigan, Maria Cantwell of Washington State, Heidi Heitkamp of North Dakota, Tammy Duckworth of Illinois and Maggie Hassan of New Hampshire, who issued statements after talking privately with each other for weeks about what to do about Mr. Franken.

Senator Richard J. Durbin of Illinois, the No. 2 Democrat, added his support, as did Senators Bob Casey of Pennsylvania, Joe Donnelly of Indiana, Patrick J. Leahy of Vermont, Edward J. Markey of Massachusetts and Sherrod Brown of Ohio, as well as Mr. Perez of the Democratic National Committee.

“It’s time for him to resign,” Mr. Durbin said. “It just seemed that the credible charges continued. I thought it might be an isolated incident or two. It seems to be that there was a pattern of conduct.”

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Serving in the United States Senate has been the great honor of my life. I know in my heart that nothing I have done as a Senator – nothing – has brought dishonor on this institution. And I am confident that the Ethics Committee would agree.

Nevertheless, today I am announcing that, in the coming weeks, I will be resigning as a member of the United States Senate.

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.

But this decision is not about me. It’s about the people of Minnesota. And it’s become clear that I can’t both pursue the Ethics Committee process and, at the same time, remain an effective Senator for them.

Let me be clear. I may be resigning my seat, but I am not giving up my voice. I will continue to stand up for the things I believe in as a citizen, and as an activist.

But Minnesotans deserve a Senator who can focus with all her energy on addressing the challenges they face every day.

There is a big part of me that will always regret having to walk away from this job with so much work left to be done. But I have faith that the work will continue, because I have faith in the people who have helped me do it.

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