President of the French far-right party and presidential candidate for the 2017 French presidential elections Marine Le Pen | Alain Jocard/AFP via Getty Images French Left’s (complex) operation to stop Marine Le Pen Left-wingers hope that by backing Alain Juppé, they will see off two political enemies.

PARIS — Thousands of French leftists will hold their noses and vote in a conservative election Sunday for what they see as a greater good: stopping ex-president Nicolas Sarkozy and keeping Marine Le Pen out of the Elysée.

According to several surveys, as many as 15 percent of participants could be left-wing sympathizers — ranging from moderate socialists to lifelong communists — will head to polling stations and vote in the first round of a conservative primary to pick a presidential candidate.

In order to vote, they will all have to stump up €2 and sign a charter stating that they share the "Republican values of the Right and the Center." Some hands will no doubt tremble before signing and writing down their name and address, as they will be required to do.

But for people like Violette Lacloche, a business owner who has always voted Left, these are exceptional times that call for exceptional measures. Far-right leader Marine Le Pen is within striking distance of the presidency. If Sarkozy wins the conservative primary, she reasons, then he will go on to face Le Pen in the final round of the presidential election next May.

And Sarkozy could lose.

That is not a risk she is willing to take. So, come Sunday, Lacloche will vote for Alain Juppé, a moderate conservative whom she sees as more likely than Sarkozy to prevail over Le Pen in 2017.

"I'd rather not be outed as a conservative voter" — A female left-wing voter

"In 2002, we (left-wingers) voted for Jacques Chirac to stop Jean-Marie Le Pen from becoming president," said Lacloche, a 39-year-old mother of two. "It's the same thing this time around, just much sooner ... We all know that the presidential election is being played out now."

Lacloche added that her father and several other lifelong Left voters in her circle also planned to vote on Sunday.

Acceptable compromise

Asked about the charter that all voters will be required to sign, Lacloche admitted she had not read it. When it was read out to her, she said she was relieved.

"There is no problem there," she said. "It states 'Republican values' — and I share those values."

She added: "In any case, voting is a compromise. Even when I vote socialist I don't support every single thing they propose, like taxes for instance, which can be against my interest as a business owner. But you vote for others, not just yourself."

Other left-wingers who replied to an informal survey conducted on Facebook also admitted they had not read the charter but were willing to bend their beliefs slightly in order to ensure that Sarkozy lost in the first round and Juppé went on to the runoff on November 27.

However, only Lacloche was willing to be cited by name. Several others justified their caution by saying that a conservative vote could be held against them in the future — even though lists of voters are not going to be made public.

"I'd rather not be outed as a conservative voter," said a female left-wing voter who answered the Facebook survey and asked not to be identified.

Prominent socialists and left-wing media personalities are particularly concerned about being seen voting for a conservative. The editor of a left-wing daily, for example, said he was worried about being photographed on his way into a polling station, as he could face accusations of hypocrisy.

Juppé's blessing

For Nicolas Sarkozy, the target of this left-wing operation, the idea that outsiders should be allowed to influence the primary is a travesty. "I will not let the Left steal this election from you!" he bellowed to a crowd of supporters recently.

His protest has limited credibility. Sarkozy was president of the Républicains party when the primary was being organized. He oversaw the nomination of an official whose role is to guarantee that the primary runs smoothly, and he signed off on the wording of the charter to be signed by voters.

"We will have to debrief about this after the election," said Sarkozy spokesman Catherine Vautrin, adding that there was no way of checking voters' political credentials at the polls.

"Nicolas Sarkozy is the best opponent for Marine Le Pen in 2017 because of the rejection he suffers from many voters and his many legal troubles" — National Front supporter

No such complaints about interference have come from the Juppé camp. The beneficiary of the left-wing mobilization has all but encouraged his erstwhile rivals to turn out on Sunday, with numerous calls to vote in "as large numbers as possible."

In a nod to the last time left-wing voters voted for a conservative to keep the far-right out of power, Juppé's campaign director, Gilles Boyer, replaced his Twitter profile photo with one of Jacques Chirac.

Boyer also tweeted a picture of a €2 coin with the remark "Weapon of mass expression" — hinting that the small sum was all that was keeping any voter from influencing the primary's outcome.

According to Yves-Marie Cann of the Elabe polling agency, left-wing interest should guarantee a high turnout for the primary on Sunday — another favorable signal for Juppé.

"We will probably see 3-3.5 million voters turn out Sunday," he said. "This is more than for the left-wing primary in 2012 and shows that people consider this to be a very important election."

National Front factor

However, left-wingers are not the only ones who plan to crash the conservatives' party.

Supporters of Marine Le Pen's National Front are also projected to turn out in droves, with 9 percent of them having expressed an interest in voting, according to Cann.

Front supporters share the same analysis of the primary as the left-wingers, but draw the opposite conclusion: In order for Le Pen to become president, they must help Sarkozy.

"Nicolas Sarkozy is the best opponent for Marine Le Pen in 2017 because of the rejection he suffers from many voters and his many legal troubles," said one National Front cadre who is close to the party's president.

In the end, the opposing effects of interference from left-wingers and National Front voters could end up canceling each other out, Cann said.

If that is the case, an unlikely frontrunner may emerge on Sunday: former prime minister François Fillon, who is now emerging as a favorite.

This article has been updated to correct the percentage of participants who could be left-wing sympathizers.