WASHINGTON — Al Franken walked onto the Senate floor just before noon Thursday and anxiously scanned the spectator galleries above to find his family. He wanted to make sure they were there in what was going to be the most difficult moment in a short but — until recent days — wildly successful political career.

Mr. Franken, a “Saturday Night Live” comedian turned progressive Democratic hero who was being mentioned seriously as a presidential candidate, was about to relinquish a Minnesota Senate seat he had barely won in the first place in the face of mounting accusations that he fondled and acted inappropriately in the past with several women.

It was an almost unthinkable loss for Mr. Franken, who continued to insist he hadn’t really done anything wrong. But if it was a personal sacrifice, it was one that could help his party in the months ahead by allowing Democrats to draw a sharp distinction between their party and its efforts to hold politicians accountable for sexual harassment and abuse and Republicans, who could be welcoming a man accused of sexually assaulting teenage girls into the Senate as early as next week.

Feelings among Democrats who demanded Mr. Franken’s resignation after another accuser came forward on Wednesday were raw as they watched a much-admired colleague step aside, and they weren’t eager to discuss the political ramifications. But privately, they said the purge of Mr. Franken — and that of Representative John Conyers Jr., the Michigan Democrat and dean of the House, this week — would stand in strong contrast with Republicans and leave their party “unassailable” on the issue.