Solere's departure was the latest in a series of defections related to pending corruption charges against Fillon.
The campaign treasurer and three lawmakers from his Republicans party announced yesterday they were quitting, amid growing pressure for Fillon to step down in favor of someone else.
Nadine Morano, a former minister who backed Fillon's candidacy, also made a U-turn today, urging the former prime minister to withdraw his bid.
"I'm among those who use their energy trying to convince Francois Fillon to step down," said Morano. "I'm worried that the French will not let us reach the second round."
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Others are also reportedly quitting the campaign, mainly those who supported staunchly conservative Fillon's more moderate rival Alain Juppe in last year's primary. Juppe was runner-up, but has said he doesn't want to run in Fillon's place.
While Fillon's popularity has nosedived in recent weeks, Juppe could qualify for the second round of the election according to an opinion poll published today.
Morano strongly criticized the plan, calling it "irresponsible" as France remains in a state of emergency because of the risk of extremist attacks.
"And the far-left will come. That will result in dramatic images," Morano said, insisting there was a serious risk of violence.