Franken -- a former comedian who made his name on the popular late-night comedy show "Saturday Night Live" before turning to politics -- has acknowledged misconduct with at least one accuser.
The 66-year-old Minnesota lawmaker apologized last month and vowed to work to regain the trust of his colleagues and voters.
But with six other women now reportedly coming forward to accuse Franken of touching them inappropriately, and the issue of sexual harassment reverberating throughout Washington, a chorus of Democratic senators said it was time for him to go.
That group include a dozen female and 17 male Democrats, among them the Senate's top Democrat Chuck Schumer, who heaped major pressure on Franken by issuing a statement saying he should step down "immediately."
It marked a dramatic and sudden show of unity against one of their own in the 100-member chamber, where the Republicans hold a slim majority.
"We must commit to zero tolerance," Heidi Heitkamp said in a tweet. "And that means Senator Franken should step down."
Two other Senate Democrats stopped short of openly demanding Franken's resignation, but said he should be held accountable and that sexual harassment should not be tolerated.
Franken's office said he would make an announcement Thursday.
"Senator Franken is talking with his family at this time and plans to make an announcement in D.C. tomorrow," said a message on his official Twitter account.
"Any reports of a final decision are inaccurate."
House Democrat Keith Ellison, a close Franken friend in the Minnesota congressional delegation, said: "I think he will do the right thing and resign."
'Moment of reckoning'
Senator Kirsten Gillibrand's Facebook post appeared to be the opening Senate salvo against Franken Wednesday, as she declared that the nation -- and Congress -- faced a "moment of reckoning" regarding sexual misconduct.
"We should demand the highest standards, not the lowest, from our leaders," she said.
"While Senator Franken is entitled to have the Ethics Committee conclude its review, I believe it would be better for our country if he sent a clear message that any kind of mistreatment of women in our society isn't acceptable by stepping aside to let someone else serve."
Wave of accusations
The pressure comes just one day after Democrat John Conyers, the longest-serving member of Congress, left the House of Representatives after several female former staffers accused him of sexual misconduct.
And it follows a wave of accusations of harassment against titans in the worlds of entertainment, the media and politics, which began with claims targeting movie mogul Harvey Weinstein earlier this year.
As a consequence of the unfolding scandals, the Senate and House both voted to make anti-harassment training mandatory for all lawmakers and staff.
The specter of sexual misconduct is also overshadowing a high-stakes US Senate race in Alabama.
Republican Roy Moore, a former Alabama Supreme Court chief justice, has been accused by several women of sexually molesting them when they were teenagers -- one was 14 at the time -- and he was in his 30s.
President Donald Trump, who himself faced sexual misconduct accusations when he was a candidate, has endorsed Moore in the closely watched special election set for December 12.
Franken was first accused last month by sports broadcaster and former model Leeann Tweeden of forcibly kissing her, and touching her without consent as she slept, during a 2006 tour entertaining US troops deployed in Afghanistan.
He had appeared determined to ride out the scandal, apologizing and saying he would work diligently to regain people's trust as he returned to Washington.
But with accusations mounting, many Democrats -- ever mindful of how the scandal could impact their efforts to gain congressional seats in next year's mid-term elections -- turned on their colleague.
The issue was pushed to the top of America's headlines again Wednesday when Time magazine named as Person of the Year a group of "silence breakers" who came forward in 2017 to publicly expose patterns of sexual harassment, assault and even rape by some of society's most powerful public figures.
Senator Bernie Sanders, who ran for the Democratic presidential nomination last year and is considered a possible Trump rival in 2020, said it was important to finally address sexual misconduct.
"But the conversation we are having now is only the tip of the iceberg," he said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)