"Senator Clinton is suffering from a stomach virus," Mr. Reines said. "She wasn't feeling well while speaking at an event in Buffalo, N.Y., this morning. She felt weak, needed to sit down and then fainted briefly. She received immediate medical attention at the site, and is now proceeding with her schedule as planned."

People at the Saturn Club said that Mrs. Clinton appeared unsteady almost from the moment she began speaking.

"She said right at the top of her remarks that she was feeling queasy, but was going to talk about Social Security reform and things like that," said Leonard R. Lenihan, the Erie County Democratic chairman. "About five minutes into the speech, she said, 'I think I'm going to sit down."'

After aides provided Mrs. Clinton with a hand-held microphone, she sat and tried to continue.

"Five minutes into that, she said: 'I think I'm going to step out for a minute and get my breath. It's warm in here, and I'm going to take my jacket off,"' Mr. Lenihan said.

When Mrs. Clinton returned to the room, she stepped back toward the podium, but faltered. She was surrounded by staff members and one took her arm. She sat down again before being helped to a back room, Mr. Lenihan said.