Facing expulsion from the New York State Legislature over accusations that he sexually harassed several female staff members, a once-powerful assemblyman, Vito J. Lopez, said on Saturday that he would resign on Monday morning.

Mr. Lopez, who had said on Friday that he would seek to remain in office for five weeks before resigning and would then run for the New York City Council, announced his new plan in a one-sentence letter to the Assembly speaker, Sheldon Silver.

“I hereby resign the public office of Member of Assembly from the 53rd Assembly District, Kings County, effective 9 a.m. Monday, May 20, 2013,” the letter said, in its entirety.

Mr. Lopez’s lawyer, who has been acting as his spokesman, did not respond to a request for comment on Saturday, and it was not clear whether Mr. Lopez, a Brooklyn Democrat, would continue to pursue his bid for the City Council.