How is Fillon still in France's presidential race?

French magistrates moved to charge the wife of French presidential candidate Francois Fillon with complicity in the abuse of public funds on Wednesday – marking a major development in an ongoing scandal that has rocked her husband's once-promising election campaign.

Penelope Fillon, 61, was paid over €500,000 from public funds as an "assistant" to her husband throughout his lengthy political career. Since the allegations first came to light in January, numerous claims have surfaced that Mrs Fillon never actually performed the job for which she was paid with from public funds.

How is Fillon's campaign holding together?

The latest development is only one in a series of scandals that have rocked Fillon's campaign in the upcoming presidential election in May. Despite a clamour of calls for him to step out of the race, he has remained adamant that he and his wife did nothing wrong.

"I will not surrender, I will not give up, I will not withdraw," Mr Fillon told reporters earlier this month, after French anti-corruption investigators announced they would be conducting a full audit of the presidential candidate's financial records.

It's neither uncommon nor illegal for French politicians to employ family members – Interior Minister Bruno Le Roux recently resigned following a scandal of his own. He hired his two daughters to work during the summer months, when parliament is not in session.

Mr Fillon is under investigation himself, and has not yet been charged. But as the case against his wife's payments goes to court, it could lead to evidence showing the presidential hopeful knowingly misused public funds in the past.

Why hasn't he stepped down?

Fillon has refused to drop out of the race despite resounding calls from within his own party, including from ex-President Nicolas Sarkozy to make a "dignified exit."

Fillon, who was the prime minister of France from 2007 to 2012, had been the front-runner in the five-way race until January, when the scandal surfaced. His approval rating has since plummeted from 54 percent last November to 17 percent in March.

His centre-right party, the Republicans, which is the largest in France's senate, has been in a panic, unable to come up with a candidate to replace him. Party leaders have reluctantly opted to back Fillon after Alain Juppe, the only other Republicans candidate popular enough to step in as a substitute, announced in early March that he would not run.

How did it get to this point?

La Canard Enchaîné, a weekly investigative and satirical journal first publicised the claims when reporters found Mrs Fillon's pay slips on the parliamentary register. The news of Mrs Fillon's assistantship came as a surprise even to staff of the National Assembly, who told reporters that she had not once set foot inside the parliament building, and was never assigned an access badge nor an official email address.

In a 2007 interview with UK newspaper The Telegraph, Mrs Fillon had also said she had "never been" her husband's assistant.

The Fillons have since insisted that the position involved legitimate work, with Penelope Fillon saying that much of the work was done from her home. But as reporters kept digging, they found that two of the couple's children had also been hired as aides – bringing the total of public funds in question up to €900,000 and sending the controversy into a spiral. Because the positions hadn't been disclosed, they were naturally viewed as suspicious.

Fillon's campaign director, his spokesperson, and several advisors have all quit following the scandal, and more than 80 high-profile supporters have since withdrawn from Fillon's campaign. Some have even switched sides to join Fillon's rival, Emmanuel Macron. Things got even worse when La Canard Enchaîné published a new report claiming Fillon received €45,000 by a Lebanese oil magnate to arrange a meeting with Russian President Vladimir Putin – with whom Fillon retains close ties.

What does it mean for the election?

The latest polling shows Fillon has little chance of winning the presidency. But he has vowed to his core supporters to hold out until the end and keep his party together.

Just months ago, an underdog among eleven candidates, Macron's self-founded party "On The Move" is now in the lead to win the presidency on May 7, projected to face off with Marine Le Pen of the National Front.

With less than a month to go before the election, Fillon is left without any major allies. Macron has now secured the valuable endorsement of outgoing Prime Minister Manuel Valls, French daily Le Monde reported Wednesday morning, leaving the long-ruling Socialist Party establishment fully backing the upstart centrist Macron to take on the far-right populist Le Pen.