Financial markets are closely watching the turbulent French campaign, in which far-right candidate Marine Le Pen is seeking to repeat the kind of anti-establishment shock that saw Britons vote to quit the European Union previous year and Donald Trump elected as USA president.
"But if they think his candidacy will finish badly, it's possible they will shift their support elsewhere - whether to [liberal centrist] Emmanuel Macron or to [far right leader] Marine Le Pen".
Paradoxically, Ms Le Pen's current legal difficulties also work in her favour.
"The thing about the judicial affairs for Marine Le Pen and the National Front is that they are not about personal enrichment, while Francois Fillon's family is directly implicated", far-right expert Cecile Alduy from Stanford University told a conference this week.
Several members of the conservative party have dropped out of Fillon's campaign this week.
"I am convinced that Francois Fillon is about to fall", Georges Fenech, a politician who led a failed rebellion against Mr Fillon last month, told France Info radio.
Accusations that Fillon gave bogus parliamentary jobs to his wife Penelope have engulfed his tilt for power.
Unlike previous French right-wing leaders who played on their staid family lives and links with the Catholic Church, Ms Le Pen's private life is unconventional.
Sources in the Fillon camp have said there was a strong push by some in the party on Wednesday to have former prime minister Alain Juppe stand in Fillon's place, but that the plan was vetoed by Nicolas Sarkozy, the former president.
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Le Pen is suspected of having signed contracts to have her cabinet chief and her bodyguard paid as European Union parliament assistants without them working in the parliament.
It is unlikely that she will be prosecuted before the first round of voting on April 23.
Replacing him as Republican nominee at this late stage would be a big gamble for the party, but having a candidate under investigation could also backfire.
In an upbeat online video message urging supporters to attend the weekend rally, Fillon said that only he could "restore France's strength".
But he said he would ask Trump to respect decades of French-US security alliances, most notably in recent times fighting the Islamic State group.
Mr Macron, a 39-year-old centrist and former economy minister, who has been criticised for being too vague on policy, laid out his agenda for Le Parisien newspaper.
This week, Mr Fillon announced that he will be summoned for questioning over the allegations known as "Penelopegate" in the next couple of weeks.
Allegations that she circulated gruesome pictures of victims of terrorism play well with her core voters, who often equate Muslims with violence.
Juppe would have 26.5 percent of votes, giving him a narrow lead over Macron on 25 percent, while Le Pen would slip to third place on 24 percent, according to the Odoxa-Dentsu Consulting poll of 943 people.