With the primaries over, the presidential campaign swings into full action in France, where political scandals, fluid allegiances and a restless electorate have thrown the contest wide open.

The three leading candidates are dogged by allegations of misusing public funds. A fourth - Benoit Hamon, who coasted to easy victory in Socialist party primaries Sunday night - faces an uphill battle rallying the weak and fractured left behind him.

"Everyone is waiting for another surprise," says political analyst Jean Petaux of Sciences Po Bordeaux University, of an election that has already upended traditional French politics. 

Today, "uncertainty" is the prevailing sentiment, Petaux says, with roughly 100 days to go before the first round of voting on April 23.

The biggest questions surround center-right candidate Francois Fillon, a supporter of traditional values and tough economic reforms, who was once considered a near shoo-in for the presidency. 

An ex-prime minister embracing a "Mr. Clean" image, Fillon is now battling for political survival following a media report that his wife Penelope was paid hundreds of thousands of euros for potentially fictional employment. 

French prosecutors are looking into the allegations. Fillon, who adamantly denies them, says he will quit the race if a formal investigation is launched.

"He's repackaged himself as the 'ethical' candidate who wants to do away with old practices of cronyism and nepotism," says political scientist Corinne Mellul of the Catholic Institute of Paris. "So it doesn't look very good for him right now."

Fighting back

On Sunday, thousands of supporters gave Fillon and his wife a standing ovation at a packed rally in northern Paris, decrying what he described as a well-timed smear campaign and warning opponents not to mess with his family.

"He's the only guy whose able to make France a better country over the next five years - we need ways to help companies expand and hire people more easily," said businessman Fadi Lahoud, 45, who attended the rally and dismissed the allegations as overblown.

Others are not so sure.

"I will not support Francois Fillon anymore" if the allegations are true, said 18-year-old university student Aurane Dibu. Instead, she will switch allegiance to ex-economy minister Emmanuel Macron, who is riding a wave of popularity as a center-left maverick. 

Macron's moment?

Indeed, a new poll published in Le Figaro newspaper finds Fillon now running neck-and-neck with Macron for second place in the first round of presidential elections in April.

Far-right candidate Marine Le Pen leads in the survey, but like previous polls, it sees her losing in a second-round runoff. More worryingly for Fillon, the poll finds Macron would not only win against Le Pen by a larger margin than he, but would beat him in a runoff between the two men.

"He has appeal with voters, especially younger voters, he's charismatic," analyst Mellul says of Macron. "But that also entails liabilities. He's not very experienced in politics, his program is not very clear. So it's not all good for him."

Still, Hamon's win is likely to bolster Macron's chances. The Socialist candidate embraces the hard-left wing of his party and his signature program, granting a 750-euro ($800) universal monthly income, is considered unrealistic. Like Fillon, Hamon was a dark-horse candidate, whose come-from-behind victory surprised pundits and the political establishment.

Hamon now hopes to forge alliances with party centrists and fellow hard-left politicians, including firebrand Jean-Luc Melenchon, who opted out of the primaries. 

A fractured left - and a hefty fine for the far-right

Yet some Socialists are already abandoning camp and aligning themselves with Macron. President Francois Hollande - who renounced a reelection bid he was likely to lose - has remained conspicuously silent.

For his part, former prime minister Manuel Valls, who lost to Macron in Sunday's runoff, called for unity in a concession speech. Yet privately he has signaled another path to his supporters, according to "Le Monde" daily: "Not to participate, play for time and see how the presidential elections play out."

Like Fillon, Macron faces allegations of questionable financing. A recently published book claims he siphoned expenses as economy minister to finance his En Marche! ("Forward!") movement. He, too, denies any wrongdoing. 

As for Le Pen, French prosecutors are probing allegations of fraudulent European Parliament payments to staff who in fact worked for her far-right party. Le Pen, who calls the allegations politically motivated, also faces a hefty, 340,000-euro EU fine.

"The allegations against Macron are nowhere near as serious as those facing Marine Le Pen and the National Front," says analyst Petaux. "That's why they've been so discreet in the Fillon affair."

France is no stranger to political scandals, and how this latest batch will affect the upcoming vote is uncertain.

"Political ethics in France is very different from Anglo-Saxon countries," says political commentator Mellul. "But France is changing. I think voters are becoming more demanding where ethics is concerned."

At the Fillon rally, 24-year-old Melodie Combot, said she was standing by her man.

"Maybe he made a mistake, but it's not proven it was illegal," Combot says of the financing allegations. "It does not change his program, it does not change his personality, it does not change his ideas."