Valerie Trierweiler posted a Twitter message supporting Socialist dissident Olivier Falorni, who is challenging Segolene Royal, the mother of the president's four children, The New York Times reported Thursday.
A parliamentary runoff Sunday will decide which candidate will represent La Rochelle in parliament.
"Courage to Olivier Falorni," she tweeted, "who has not been unworthy, who has been fighting unselfishly alongside the people of La Rochelle for so many years."
Royal said she was "mortified" by the posting and asked for respect on behalf of her children, the Times reported. Prime Minister Jean-Marc Ayrault advised Trierweiler to remember her place and be more "discreet."
Trierweiler, a journalist, had previously said she would make a better first lady than the previous president's wife, Carla Bruni, Britain's The Daily Telegraph reported.
"Carla Bruni comes from a world entirely alien to politics: fashion, show business. She doesn't know its codes," Trierweiler said. "I know politics, I know the media."