Fillon was charged in March with misuse of public money and corporate assets, mainly over the employment of his British-born wife Penelope as a parliamentary assistant for 15 years.
It was a severe blow to the 63-year-old former prime minister, who clinched the nomination for the Republicans party in November by claiming he was unsullied by the scandals that surrounded his rival and former boss, ex-president Nicolas Sarkozy.
The allegations that Penelope had done little work to earn 680,000 euros ($725,000) were first reported by Le Canard Enchaine newspaper in January.
Fillon's reaction to the accusations has been to deny that either he or his wife have done anything wrong and to claim his left-wing rivals are operating a "secret cell" to blacken his name.
It is a line of attack that has drawn scorn from Socialist President Francois Hollande and surprised even some of Fillon's own allies.
Having backtracked on an early promise to withdraw his candidacy if he was charged, the erstwhile arch-conservative has found himself in the unlikely position of running as an anti-establishment rebel determined to defy the media, the government and magistrates he says are working against him.
Subsequent revelations that a wealthy Franco-Lebanese lawyer bought handmade suits for Fillon worth 13,000 euros each have drawn further ire from his opponents.
The charges have caused Fillon to slip from the position of clear frontrunner he occupied early this year.
Yet despite the charges, support for him is still hovering at around 19 percent, putting him tied for third in the race with the hard-left Jean-Luc Melenchon but behind co-leaders Marine Le Pen of the far-right and centrist Emmanuel Macron.
Some observers believe Fillon is under-estimated in opinion polls and might perform far more strongly in the first round on April 23 than they show.
One of France's top pollsters, Jerome Fourquet of IFOP, said while many right-wing voters were "highly disappointed" by the scandal surrounding Fillon, "it is not impossible that part of this electorate returns to his camp at the last moment".
Cuts and competitiveness
Fillon's policies are based on deep cuts in public spending and slashing hundreds of thousands of jobs from what conservatives believe is a bloated civil service.
He also wants to attack one of the sacred cows of the French left, the 35-hour working week, raising it to 39 hours.
A leaner, meaner France could, he claims, rival Germany as the foremost economy in the eurozone within a decade.
In TV debates, Fillon has stressed that only he among the candidates has experience of running a country.
Supporters have also cheered his hardline stance on immigration.
Fillon raised eyebrows when he told immigrants in a TV debate that "when you enter someone else's house, you do not take over."
His outspokenness stood in contrast to his image as prime minister, of a quiet and urbane man whose steady temperament contrasted with the impulsive Sarkozy who once dismissed him as "Mr Nobody".
Once the youngest member of parliament at age 27, Fillon is a devout Catholic who voted against gay marriage when it was legalised in 2013.
The self-declared "Gaullist" -- a form of nationalism that proposes an independent and strong France -- also has a close bond with Russian President Vladimir Putin.
The two men overlapped as prime ministers from 2008-2012 and their closeness has led to questions about Fillon's foreign policy.
Country manor house
Fillon and his Welsh wife met at university in France when they were in their early twenties.
They soon married and live in an imposing manor house near Le Mans in northern France where they brought up their five children.
Two of their children have also had paid work for their father in parliament, performing roles as "legal advisors" despite not being qualified lawyers at the time.
Penelope Fillon was until recently a low-key political wife, a keen horse-rider who once described herself as a country "peasant" who preferred the countryside to Paris.
In examining Fillon's insistence that his wife has "always" worked to help his career, the French media has honed in on previous comments she made.
"Until now, I have never got involved in my husband's political life," Penelope told regional newspaper Le Bien Public last year, echoing a similar statement to a British newspaper in 2007.
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