PARIS — When France sneezes, Europe catches pneumonia.

Rarely have the French faced a starker choice of European futures than in this year’s presidential election, where they’ll vote for “bye-bye European Union” or “back to a Europe of Nations” or even “fast forward to a more integrated Union.”

If the opinion polls are accurate and fickle voters, angry with the political class, don’t change their minds at the ballot box, the second-round runoff on May 7 is likely to pit anti-EU nationalist Marine Le Pen against pro-EU centrist Emmanuel Macron. Still in contention in the first round on April 24 are conservative Gaullist François Fillon and evergreen leftist Jean-Luc Mélenchon.

Le Pen has promised to take France out of the euro and the Schengen open-border zone, reintroduce the franc and reimpose border controls and trade barriers, and has said she will call a referendum on leaving the European Union altogether within six months if the bloc isn’t receptive to her proposals for a radical treaty renegotiation.

It is no exaggeration to say that a Le Pen victory could deal a fatal blow to the eurozone and the EU, which can survive a Brexit but would be mortally wounded by a Frexit. Germany’s federal election on September 24 matters too, but is less uncertain — the German vote is bound to produce another coalition led by one or the other pro-European mainstream party. For now, all eyes are on Paris.

“Europe is locking us up, Europe is forbidding us, Europe is bullying us” — Marine Le Pen

France, unlike Britain — which joined the EU late and was always semi-detached — has been a central pillar of European construction from the outset. Its passions and tantrums have dictated the tempo and shape of integration.

From the failure of the European Defense Community to the Treaty of Rome, from De Gaulle’s vetoes of Britain’s first membership bids to the “empty chair” crisis and the “Luxembourg compromise;” from the creation of the European Monetary System and the euro to the rise and fall of the proposed European constitution, the European project has advanced or retreated at France’s pace.

The result of the May 7 vote, and of subsequent parliamentary elections in June, will determine whether the EU is plunged into existential crisis or given a new lease of life with the prospect of a Franco-German grand bargain to deepen European economic, monetary and defense cooperation.

Neither French front-runner can necessarily count on a parliamentary majority to implement their program and their promises may well be tempered by the influence of the political and business establishment.

Yet in the wake of Britain’s vote to leave the EU and the election of Donald Trump in the United States, the choice between isolationism and internationalism, between a closed, protectionist, angry France and an open, economically liberalizing, optimistic one is a litmus test that will shape Europe’s future for years to come.

“Europe is locking us up, Europe is forbidding us, Europe is bullying us,” Le Pen declared during a television debate among the main candidates last month. “I don’t aspire to administer what would have become a mere region of the European Union. I don’t wish to be vice chancellor to [German Chancellor Angela] Merkel.”

Aware that her plan for an economic and monetary leap into the unknown scares the middle-class voters she needs to win over to reach 50 percent in the runoff, Le Pen has kept quiet about leaving the euro, seeking, instead, to project a reassuring “I’m-on-your-side” image and appear stateswoman-like by meeting Russian President Vladimir Putin or addressing the parliament in Chad.

Perhaps surprisingly, given the current grumpy, Euroskeptical mood in France, polls predict the most pro-European candidate, 39-year-old favorite Macron, has the best chance of beating the far-right leader in the runoff. This despite his avowed support for pooling more sovereignty in the eurozone and building a European defense union in close partnership with Germany.

In one of the rare dramatic exchanges in this week’s marathon TV debate among the 11 candidates, he accused Le Pen of proposing “economic war.”

Macron, who was top economic adviser and then economy minister under Socialist President François Hollande before resigning last year, is the only candidate who has promised to adhere to France’s much-neglected European obligation to bring its budget deficit below 3 percent of gross domestic product. His commitment to fiscal responsibility and economic reform, and his sudden rise to front-runner, won him an audience with Merkel last month after the center-right German chancellor had declined to make time for him — probably out of tribal loyalty to her French conservative allies — when he visited Berlin in January.

The leader of the grassroots movement En Marche is taking political risks — not only by embracing “more ambitious European integration” but also by hailing Merkel’s decision to welcome roughly 1 million refugees and migrants in the summer of 2015 as a move that saved Europe’s “collective dignity.” Merkel’s open-door policy has been widely criticized in France, where Hollande accepted EU’s mandated quota of refugees as quietly and unenthusiastically as possible and was relieved when it turned out that few refugees came.

Macron is backed by federalists including liberal MEP Sylvie Goulard, who accompanied him to Germany, and former Greens leader Daniel Cohn-Bendit. He chose Berlin’s Humboldt University, where former German Foreign Minister Joschka Fischer called for a “European federation” in a celebrated speech in 2000, to issue his own call for a eurozone budget with a borrowing capacity to fund public investment and help protect countries hit by economic shocks, as well as a European defense fund to finance military research and joint arms procurement.

The third man in the race, Fillon, offers a more sovereignty-focused approach to the EU, saying in Tuesday’s debate, “It’s time for France to take back the controls of the European Union.”

But, dogged by successive scandals for using taxpayers’ money to employ family members as political aides, Fillon has struggled to gain traction with his vision of a more inter-governmental Europe of nation states.

Fillon has vowed to shrink the regulatory intrusion of the European Commission and put a directorate of national leaders in charge of the eurozone, taking fiscal supervision away from the EU executive. He backs a powerful Franco-German axis “because without a strong entente between our two countries, there would be no Europe, and our continent would be open to the Americans, the Russians, the Chinese.”

Firebrand leftist orator Mélenchon has said he would demand a complete negotiation of the EU treaties to put an end to fiscal austerity.

Merkel has been publicly reserved about Fillon — especially since the scandal broke, the day after his visit to Berlin, an occasion he used to criticize her immigration policy. A German source said the chancellor was impressed by the determination he expressed to roll back the frontiers of the state and face down the trade unions but was aware of the resistance to reform in France.

Meanwhile, the champions of the divided French left — with little to no chance of surviving the first round of elections — vie with utopian visions of a reinvented free-lunch socialist Europe.

Firebrand leftist orator Mélenchon has said he would demand a complete negotiation of the EU treaties to put an end to fiscal austerity, make the European Central Bank print money to fund investment and write off government debt, and impose a European “protectionism of solidarity” to shield jobs. Asked how he would convince other Europeans, notably the Germans, to accept such change, he told an audience this week, “We can afford to create a balance of forces and threaten to walk out, because we are not [Prime Minister Alexis] Tsipras’ Greece, we are France.”

Ultimately, the European future that emerges from France’s ballot boxes may hinge above all on the next president’s ability to revive the economy with reforms that restore a more balanced partnership with Germany.

Paul Taylor writes POLITICO's Europe At Large.