Le Pen has already seen her earnings as an MEP cut because of alleged misuse of European Union funds.
"I submit myself before the French people, because only the ballot box, and not an investigation directed against me, can decide who will be the next president of the republic", he said on Wednesday.
However, cracks within his own party quickly appeared, as one of his campaign officials, Bruno Le Maire, bowed out without hours, saying he can't follow a man who won't honor his word to withdraw if charged. She's very close. She's leading the race.
"Daesh is THIS!" Le Pen wrote, in a series of vitriolic tweets that included photographs of killings by IS in late 2014. In this way she tried to draw the public's attention to the cruelty of extremists. As such the litigation will not prevent Le Pen from participating in the election but may harm her image. France would default on its sovereign debt if it unilaterally converted its euro-denominated obligations into new francs following a National Front election victory, Moritz Kraemer, S&P's head of sovereign ratings recently wrote.
In response angry Le Pen Tweeted photos showing fiendish executions committed by Islamists.
The 63-year-old Fillon said he was trying to turn the page on a nightmare week that had seen scores of defections from his camp after he admitted he faces criminal charges over allegations he gave his family fake parliamentary jobs.
The financial prosecutor's office, which has been investigating Fillon since January 25, decided Friday to launch a formal judicial inquiry.
In February, the politician admitted to setting up a £6,000-a-month fake job in the EU parliament for bodyguard Thierry Legier, according to the European Anti-Fraud Office.
Body of missing boy found in Murray River, mother in custody
The 27-year-old mother with a troubled history seems more known about than really known in the town of 8000 people. The father of the boys has spoken of his devastation and said he was numb after learning about what had happened.
With the governing Socialists trailing in polls and with a newcomer to politics, independent centrist Emmanuel Macron, sharing the lead with Le Pen, France's traditional left-right divide could be obliterated this year, upending politics as usual.
Ultimately, however, Ms Le Pen's biggest asset is that all her current presidential competitors are weak.
Mr Fillon's decision to cling on also sharpens the thorniest question for his party.
Global trends also favour Ms Le Pen.
New opinion polls continued to show Fillon would fail to make the second round of the April/May election, albeit by a narrow margin. This incident took place when Macron visited an agricultural fair in Paris, which is a known campaign stop for presidential candidates in France.
If nobody achieves that, the two top candidates go into a second round, with the victor being decided by a simple majority. However, she would lose the run-off in May to whoever should be her opponent.
However, Le Pen, now the most popular candidate in the French presidential elections but unlikely to win a second round vote under France's electoral system, is unlikely to be hurt politically by any investigation.