Clinton, 57, was speaking at the Saturn Club in Buffalo, said Erie County Democratic Chairman Len Lenihan, who greeted her upon her arrival.
"She said she was not feeling well, looked pale," CNN quoted him as saying. "She told the crowd she was queasy.
"She said she was going to sit and talk." She took the microphone but then said she felt warm and left the room to remove the jacket of her black jumpsuit.
However, on her return, the senator fainted as her staff placed the microphone in front of her, Lenihan said.
Her Secret Service agents quickly surrounded the former first lady, and four people broke her fall, he said. "They took her down gracefully; let's put it that way," said Lenihan.