And it has all happened at the start of a tax-cut fight that Franken was supposed to help lead.
The Democrats' multilevel campaign to stop Republican-backed tax cuts, which has included ad buys, news conferences and activist pressure, was dealt a blow by Franken essentially being forced to take the bench. The senator had taken an increased, unique role in Democratic messaging this year, with a best-selling memoir, frequent use of social media and a new openness to national interviews.
"You have to be able to hit back hard, quickly and in a way that works, and he's uniquely skilled at doing that," said one Senate Democratic aide, who spoke on the condition of anonymity to be candid.
The aide conceded that Franken's diminishment was a setback but suggested that "there are other senators who are doing their jobs who are just as good at this."
But the aide did not say who. Franken, whose fame as a comedian predated his service in the Senate, capitalized on his name recognition this year to become one of the party's most effective fundraisers. When he signed a Democratic fundraising email, Franken's name produced more donations than anyone else in the 48-member Senate caucus, said Democrats with knowledge of their party's fundraising efforts.
On Thursday afternoon, Democratic Sens. Jon Tester, Mont., and Tammy Baldwin, Wis., announced that they would give money from Franken's PAC to charity.
Franken had also been delivering for Democrats with viral videos, teaming up with celebrities (David Letterman) or colleagues (Elizabeth Warren). Last week, a five-minute video Franken and Warren recorded to argue that Republican tax cuts would not lead to CEOs hiring more U.S. workers was viewed more than a million times on Facebook.
On Thursday, Warren said in a statement that Franken's behavior in 2006 - broadcaster and model Leeann Tweeden said he "forcibly kissed" and groped her during a USO tour - had been "unacceptable and deeply disappointing," and agreed with his decision to face an ethics investigation.
Franken's videos stood out, in part, because the coalition of progressives working to defeat the GOP tax-cut legislation have struggled to recover the energy from the fight to save the Affordable Care Act. The town hall meetings of spring and summer have generally petered out; just four Republican members of the House and one senator have scheduled town halls over next week's pre-Thanksgiving recess.
The #NotOnePenny coalition has kept up the pressure anyway, with a small Wednesday rally near the Capitol and in-state lobbying planned for the coming week. The addition of partial ACA repeal to the Senate's version of the tax bill had helped galvanize activists, they said.
"We're going to find senators and members of Congress who voted for this legislation, and we're going to hold them accountable - office visits, protests, rallies," said Tim Hogan, a spokesman for Tax March, a progressive group organizing against the Republican legislation. "The more people talk about the contents of these bills, the harder it gets for Republicans to sell it and to hide from the consequences of their actions."
Franken had been trying to help with that pitch. "Republicans are trying to pass a terrible tax bill that would increase the deficit and cripple the Affordable Care Act," he said in his final Facebook video before the scandal broke. "It was millions of Americans, including many of you, that made their voices heard and helped kill Trumpcare."
But by the end of Thursday, even some members of the coalition were holding Franken at arm's length. MoveOn released a statement criticizing him; the group's Washington director, Ben Wikler, formerly worked for Franken. Justice Democrats, a progressive group that backs primary challengers to incumbent Democrats, called on Franken to resign and be replaced by Rep. Keith Ellison, D-Minn.
"We believe Leeann Tweeden's account and are disappointed in Senator Franken's actions," the Progressive Change Campaign Committee, a group that had teamed up with Franken before, said in a statement. "Sexual harassment and assault are not joking matters. This behavior isn't excusable in any context, and we're glad he's attempting to take responsibility and show remorse. We hope he commits himself to being part of a necessary culture shift in the halls of Congress, the entertainment industry, and across society."