A blow for French President Emmanuel Macron on Friday.
A close lieutenant, Benjamin Griveaux, has pulled out of the race to become Paris mayor.
It's been alleged he sent sexual images to a woman who is not his wife.
Griveaux said he and his family faced "ignoble" attacks online.
(SOUNDBITE) (French) FORMER PARIS MAYORAL CANDIDATE, BENJAMIN GRIVEAUX, SAYING:
"For more than a year, my family and I been subject to defamatory speech, lies, anonymous attacks, disclosure of stolen private conversations, as well as death threats. This torrent of abuse hurt me, but above all it has hurt my loved ones."
A Russian dissident artist has published screenshots of online "sexting" between Griveaux and a woman, saying it "exposed his hypocrisy."
Griveaux hasn't denied sending the messages.
At 42, he is one of the so-called "Macron boys," a clique that helped propel the former investment banker to the Elysee Palace.
He was central to Macron's plan to control the capital and build a Parisian power base for his En Marche party.
Griveaux was already struggling, though. Recent polls put him in third place.
The incumbent mayor and a leading rival have backed Griveaux in the scandal row, saying his privacy should be respected.