Conservative candidate Francois Fillon has fallen from front-runner to struggling survivor in France's presidential race as corruption allegations have damaged his image.

Fillon's campaign has been flailing since the investigation began in January. He was handed preliminary charges last month, including misuse of public funds, and receiving money from misuse of public funds and company assets.

The 63-year-old politician denies any wrongdoing, and opinion polls suggest he has a chance to be one of two candidates to advance to the May 7 runoff election.

Fillon hopes his experience as former prime minister and hard-line views on security issues will give his campaign a boost following the shooting of police officers Thursday in Paris.

He initially said he would withdraw from the race if he was charged, but later said he was determined to let voters judge him instead of investigators.

"France is greater than my errors", he said, calling the judicial investigation a "political assassination."

Fillon, who was prime minister from 2007 to 2012 during Nicolas Sarkozy's presidency, says his priority as president would be to fight "Islamic totalitarianism."

He has pledged to boost police and military forces, cut taxes on businesses, slash public spending to boost France's stagnant economy and to reduce immigration "to a strict minimum."

He also promises to drastically reduce the number of public servants, raise the retirement age from 62 to 65 and extend the workweek beyond 35 hours.

He has a strong focus on traditional family values. He pledged to ban full adoption by same-sex couples.

Since January, he was forced to limit his campaign events to political rallies and a few visits under high security, to avoid anti-corruption protests and demonstrators shouting "Fillon in prison!"

In 1981, Fillon was elected to parliament for the first time, representing his electoral fief of Sable-sur-Sarthe, a small town in rural western France.

At 27, he was the youngest lawmaker of the lower house of parliament, the National Assembly.

He served five times as a government member under Francois Mitterrand and Jacques Chirac.

In the wake of the global economic crisis in 2008, Fillon as prime minister implemented a plan to cut public spending and raise taxes.

The corruption scandal has his Welsh wife, Penelope, under the spotlights. She was previously seen as a discreet housewife, the mother of five, and not involved in politics.

Fillon is suspected of paying Penelope and two of their children more than 1 million euros ($1.1 million) over many years for positions as parliamentary aides that involved no sustained work.

It is legal, and not uncommon, for French lawmakers to hire family members or close acquaintances as parliamentary aides — but they are expected to work.

Judges are also looking into allegations that Fillon was given suits by lawyer Robert Bourgi, an unofficial adviser for French and African presidents, worth more than 48,000 euros ($52,000) over the past five years — including two suits with a combined value of 13,000 euros ($14,000) last month.

Bourgi had close links to former French presidents Jacques Chirac and Sarkozy, as well as the former leaders of Senegal and Gabon.

Judges are also investigating whether Fillon and his wife committed fraud and forgery in a cover-up attempt.