A leading French party on Saturday suspended 66-year-old Senator Joel Guerriau after his arrest for giving a date-rape drug to a female MP as part of a plot to carry out a sexual assault.
Guerriau is accused of spiking a glass of champagne he gave to 48-year-old Sandrine Josso, whose lawyer said she was now in "shock".
The senator and banker was formally charged on Friday and is now in detention, while revelations over the case have shocked the French public.
The Horizons party, led by former prime minister Edouard Philippe, suspended Guerriau on Saturday as a first step to kicking him out.
Horizons said it "will never tolerate the slightest complacency towards sexual and sexist violence".
The Independents caucus in the French Senate also suspended Guerriau and warned he would be expelled.
Josso, a member of the centrist MoDem party, fell ill after accepting a glass of champagne on Tuesday night at Guerriau's Paris home, prosecutors said.
The two have known each other for about 10 years but are not in an intimate relationship, they added.
She saw the senator "grabbing a small plastic bag containing something white, in a drawer in his kitchen", her lawyer Julia Minkowski told AFP.
"She had to deploy monumental physical and intellectual forces to overcome her terror and extricate herself at the last minute from this ambush," Minkowski said.
Police searched Guerriau's office and home and prosecutors confirmed that ecstasy had been found. Tests revealed that Josso had ecstasy in her system, investigators said, prompting her to file a criminal complaint.
Guerriau, a senator since 2011, denied any sexual assault plot in a first formal questioning on Friday.
A source close to the investigation said Guerriau had told investigators he believed that he had procured a drug to induce euphoria, but that it was not ecstasy, from another senate member to help him with personal troubles.
Guerriau "will fight to prove he never intended to administer a substance on his colleague and longstanding friend to abuse her", said his lawyer, Remi-Pierre Drai.
"Guerriau is not a predator. He is an honest man, respected and respectable."
Josso has received widespread backing. "Full and complete support to my colleague Sandrine Josso," said MoDem vice president in the National Assembly, Elodie Jacquier-Laforge, in a social media post.
"Neither procrastination, nor complacency, our thoughts are with the complainant," said Christophe Bechu, minister for ecological transition and secretary general of Horizons.
Saint-Sébastien-sur-Loire, the western town where Guerriau was mayor for 22 years, was "in shock", according to local lawmaker Herve Camus.
He said Guerriau was an "affable" man who had a "certain aura" with the town's population but who could also be "very unpleasant with opposition lawmakers".
Laurent Keunebroek, an opposition lawmaker in Saint-Sebastien, said the scandal could have "consequences" on the views about elected officials.