Fillon's campaign has been struggling since it emerged that his Welsh-born wife Penelope was paid 830,000 euros as a parliamentary assistant over more than a decade -- despite almost no one recalling her on the job.
On Thursday, one of France's main investigative news programmes, Envoyé Spécial, aired previously unseen footage from a 2007 interview in which she said, referring to her husband: "I've never been actually his assistant."
The candid remarks in the interview with Britain's Sunday Telegraph were likely to compound suspicions that she was paid for a fake job.
The Canard Enchaîné newspaper has unearthed payslips showing she earned thousands of euros a month from 1998 to 2007 and again in 2012, as an assistant to Fillon and his 2002-2007 replacement in parliament.
Her lawyer, Pierre Cornut-Gentille, insisted her remarks had been "taken out of context."
Fillon, a conservative who had been leading the race before "Penelopegate", has repeatedly denied any wrongdoing.
But an Elabe poll on Wednesday showed his campaign to be in deep trouble.
The survey showed he would crash out in the first round of the election in April behind far-right leader Marine Le Pen and fast-rising 39-year-old centrist Emmanuel Macron.
The poll showed Macron going on to easily defeat Le Pen in May's runoff.
On Thursday, a combative Fillon ploughed on, using a rally in the northeastern town of Charleville-Mezieres to lash out at the left, which he accuses of being behind the revelations.
"They are not looking to see justice done but to take me down and, beyond me, take down the right and steal its vote," he accused.
But many conservatives are dismayed by the scandal, fearing it could cost them an election they expected to handily win after five years of troubled Socialist rule.
"It's scandalous that his wife received such huge sums of money," said Anne Serise-Dupuis, a 66-year-old Republicans voter in the southwestern city of Bordeaux. "He should withdraw. He has been disqualified."
A Harris Interactive poll showed a majority of Republicans supporters still having faith in Fillon, however.
Alain Juppé as Plan B?
It is not illegal in France to hire family members as parliamentary assistants -- provided they have real jobs.
Fillon, 62, claims his wife, whom he previously portrayed mainly as a home-maker, looked after much of his constituency business in the central Sarthe region while he was in Paris.
This week it emerged that he also had two of his five children on the payroll.
Investigators this week raided his parliamentary office and interviewed the couple as part of a preliminary probe into possible misuse of public funds.
Penelope's lawyer said she had handed over "all the details proving the existence of a real job".
The allegations are all the more damaging given that Fillon, a devout Catholic, campaigned as a sleaze-free reformer who would slash public spending to stop France living beyond its means.
A group of leading Republicans said Thursday they would stand by their man.
"We give François Fillon our complete support because his commitment to France is vital," the group, which included former finance minister François Baroin, touted as a possible replacement for Fillon, wrote in the rightwing Le Figaro daily.
But some dissident voices have already emerged.
Republicans MP Philippe Gosselin said he and other party members would call on 71-year-old Bordeaux Mayor Alain Juppe -- whom Fillon beat in a primary -- to step in if Fillon decided to stand aside.
Meanwhile, the surprise winner of the Socialist nomination, Benoît Hamon, met President François Hollande as he searches for support within his fractured party to mount a credible challenge to Le Pen, Macron and Fillon.
Hamon, a 49-year-old leftwinger with a radical proposal to pay a universal basic income in a world of dwindling work, will probably need to win over the votes of Communist-backed Jean-Luc Melenchon to stand any chance.