Far-right candidate Marine Le Pen has taken the lead in opinion polls and centrist Emmanuel Macron has also benefited from Fillon's woes.
Now polls suggest Mr Macron, who has formed a movement called En Marche!, will contest the decisive second round of the election on May 7 with his likely opponent Marine Le Pen of the far-right National Front in a race that has already confounded the predictions of pollsters and pundits.
But French prosecutors requested a waiver past year after she tweeted three images of Islamic State executions, including.
Polls suggest Macron and Le Pen may face off in the May 7 presidential runoff.
More than 60 politicians have said they could no longer support Fillon, who is set to be charged for the embezzlement of public funds despite his protests of innocence. "It's the presidential election", he said, flanked by senior party members, after a morning of speculation he was about to quit the race. "I will not pull out", he said.
With the March 17 deadline for the collection of signatures necessary to launch a candidacy fast approaching, a member of Juppe's entourage told AFP he was prepared to take over - under certain conditions.
But he has since been hit by a series of allegations that he paid his British-born wife Penelope and his children hundreds of thousands of euros since 1988 for allegedly fake parliamentary jobs.
Le Pen was called in for questioning for wrongfully claiming aides' salaries from the European Parliament.
A poor showing at the support rally would impact Fillon's legitimacy, which has been based on his win in the primary held by conservatives and centrists.
While Ms Le Pen had intended the images would illustrate the difference between a Western European political party and a murderous Islamist regime, they instead landed her in hot water as publishing such images in France is against the law.
Le Pen's chief of staff, Catherine Griset, was handed a preliminary charge of receiving money through a breach of trust.
Fillon appeared increasingly alone with the resignation of two key officials, and other high-profile supporters following them out.
And so he knows, when he unexpectedly cancels an appearance at one of the calendar's most important electoral events and calls a last-minute press conference, it will grab attention: he knows it's the knife that France is watching. A host of polls show the National Front candidate winning the first of France's two-tiered presidential election.
As Fillon's fortunes fell, former Prime Minister Alain Juppe again was being mentioned as a potential Plan B for The Republicans party, which could find itself without a candidate as the April 23 first round of the presidential election nears.
He has capitalised on it heavily, unveiling a string of policies to clean up French politics including a ban on MPs employing family members.
However, Le Pen, now the most popular candidate in the French presidential elections but unlikely to win a second round vote under France's electoral system, is unlikely to be hurt politically by any investigation.