Senator Al Franken is facing calls to resign over inappropriate behaviour towards a radio host in 2006, as the scandal over politicians' sexual misconduct spreads to the Democratic Party.
Leeann Tweeden, a broadcaster and model, said Mr Franken forcibly kissed her during a rehearsal for a sketch delivered to armed forces stationed in the Middle East, and groped her while she slept on the return flight, in a moment turned out to have been caught on camera.
Mr Franken, who rose to prominence as a writer and performer on the TV show Saturday Night Live, became a US senator in 2009, representing Minnesota. He acknowledged the incident and apologised. "I don't know what was in my head when I took that picture, and it doesn't matter. There's no excuse. I look at it now and I feel disgusted with myself. It isn't funny. It's completely inappropriate. It's obvious how Leeann would feel violated by that picture. And, what's more, I can see how millions of other women would feel violated by it," he said.
Politicians across the political divide condemned his behaviour, with some calling for him to resign and others arguing that he should face the Senate ethics committee. The Democratic leader in the Senate, Chuck Schumer, said that "sexual harassment is never acceptable and must not be tolerated".
Some Democratic commentators have also raised the sexual misconduct of the former president Bill Clinton in the 1990s, as Democrats confront their own history of sexual misdemeanours.
President Donald Trump, who himself faced allegations of sexual harassment and was caught on tape that emerged during the presidential campaign boasting about assaulting women, took to Twitter to denounce Mr Franken. The photograph of the senator with Ms Tweeden "speaks a thousand words", he said, adding: "To think that just last week he was lecturing anyone who would listen about sexual harassment and respect for women."
The controversy comes amid widespread allegations against Roy Moore, the controversial Alabama judge who is contesting a Senate election next month. Several women have come forward to accuse him of initiating sexual contact with them when they were teenagers and he was in his 30s.
The Senate hopeful has defied calls to withdraw from the election despite the Republican leadership urging him to step down. Mr Trump has yet to comment on the case.
The Franken allegations come as Congress confronts accusations of pervasive sexual harassment on Capitol Hill, with several unnamed women alleging inappropriate sexual advances in media interviews this week.
At a hearing in Congress, the Californian representative Jackie Speier said she knew of two Congressmen, one Democrat and one Republican, who had sexually harassed staff members.
Speaker Paul Ryan announced in the wake of the hearing that the House of Representatives would initiate anti-sexual-harassment and anti-discrimination training for all members and staff. "Our goal is not only to raise awareness but also make abundantly clear that harassment in any form has no place in this institution," he said.