Defying forceful demands for his resignation, Representative Anthony D. Weiner of New York said on Saturday that he was entering a psychological treatment center and seeking a leave of absence from the House to deal with a pattern of reckless online behavior with women.

The announcement came as three top Democratic leaders declared that Mr. Weiner, 46, once a rising star in the party, needed counseling and should step down from Congress.

Pressure on Mr. Weiner to leave the House, and spare the Democratic Party from an increasingly embarrassing scandal, had been building all week, but intensified on Friday, after it was revealed that Mr. Weiner had traded private messages with a 17-year-old girl in Delaware.

Mr. Weiner, who friends say has become distraught and fragile in recent days, will use the leave to think about his future and whether to leave Congress, a possibility he has not entirely ruled out, a person close to him said.