WASHINGTON — Support for Al Franken all but collapsed on Wednesday among his Democratic colleagues in the Senate, with dozens calling for him to resign after a sixth woman said he had made an improper advance on her.

“Senator Franken should resign,” Senator Chuck Schumer of New York, the Democratic leader, said Wednesday evening, the latest in an avalanche of statements that began with a half-dozen Democratic women and then snowballed throughout the day. “I consider Senator Franken a dear friend and greatly respect his accomplishments, but he has a higher obligation to his constituents and the Senate, and he should step down immediately.”

Mr. Franken, Democrat of Minnesota, has scheduled an announcement on his future in the Senate for Thursday, and he pushed back on a Minnesota Public Radio report that he would be resigning. “No final decision has been made and the Senator is still talking with his family,” his office said on Twitter.

By Wednesday evening, there was widespread expectation among senators in the Democratic caucus and aides that Mr. Franken would step down. If he does, he would be the most prominent lawmaker so far to be felled by the swirling allegations of sexual harassment and misconduct sweeping through the Capitol.