That said, polls show that Le Pen is still a long shot for victory in the second round of voting, with Emmanuel Macron also poised to benefit - at least in the short term - from Fillon's woes.
"His image has been seriously damaged, and what's worrying for him is that even among his supporters many are not convinced," said Yves-Marie Cann, director of political studies at pollster Elabe in Paris.
"The fire is not contained." Cann said.
it's almost certain the next round of polling will show Fillon has lost support, though it's too early to judge whether the main beneficiary will be independent centrist Macron or the anti-European Union Le Pen.
Fillon's initial attempts to explain his way out of the growing scandal were deemed unconvincing by three-quarters of the French, a poll said.
"Francois Fillon is innocent, but the media pressure is such that the French have condemned him," Fillon's spokesman Benoist Apparu said LCP television. "It's totally unjust."
Fillon's campaign team and the Republican Party declined requests for comment.
Fillon has promised to withdraw from the race if the preliminary inquiry becomes a formal one - something that has never happened to a major French candidate this late in a presidential contest.
The former Prime Minister has been the establishment's leading candidate in the battle to stop Le Pen's populist campaign to seize the presidency and lead France out of the euro. He won the Republicans' primary boasting that he had the irreproachable integrity required to lead the country.
While polls show Le Pen leading in the first round of voting, they've recently shown Macron close on Fillon's heels for the April 23 first round of voting. Surveys also suggest that Le Pen would lose heavily to either Macron or Fillon in the May 7 run-off.
A Kantar Sofres poll released in Le Figaro showed about 22 per cent of voters now back Fillon for the first round of voting, leaving him just one point ahead of Macron and three behind Le Pen. Before the inquiry, Fillon was considered the favourite to be France's next president.
Investigators searched for documents at the National Assembly, a day after Fillon and his wife were separately questioned for about five hours by the police as part of the probe.
The presidential candidate was "happy to defend his honour," Antonin Levy, Fillon's lawyer, said on BFM television. A parliamentary job's assistant is often "less tangible"than other types of work, Levy added.
Marc Joulard, who replaced Fillon at the National Assembly, was also questioned, Agence France-Presse reported. Investigators have searched the offices of Revue des Deux Mondes, the magazine that Penelope Fillon contributed to, and interviewed the magazine's former director and its owner, billionaire Marc Ladreit de Lacharriere.
- Bloomberg
