Emmanuel Macron delivers a speech during a campaign rally on May 1 in Paris. (Eric Feferberg/Agence France-Presse via Getty Images)

POLLS SHOWING that centrist Emmanuel Macron will comfortably win the French presidency in a runoff with far-rightist Marine Le Pen on Sunday probably are more reassuring than they should be. First, an upset is still possible: Ms. Le Pen’s voters tend to be more motivated than those of Mr. Macron, and many on the French left may stay home, having judged a neo-fascist xenophobe and a former investment banker to be equally offensive. Some on the traditional right may also defect to Ms. Le Pen: Shamefully, Pope Francis and the Catholic Church have declined to take a stand against her, despite her anti-Muslim demagoguery.

More fundamentally, even a comfortable win for Mr. Macron would merely paper over the fact that popular support for values such as religious tolerance, and for French participation in Western institutions such as the European Union and NATO, is alarmingly attenuated. In the election’s first round, just short of half of voters chose candidates outside of that political mainstream. Three of the top four were favorably inclined toward Vladi­mir Putin.

If Mr. Macron, a self-described radical centrist and unapologetic defender of the European Union, nevertheless emerges triumphant, it will be in part because he managed to position himself as another outsider battling the establishment. Though he served as economy minister under the deeply unpopular outgoing president, François Hollande, Mr. Macron quit and founded his own political party a year ago. He has crusaded against the sclerotic statism that has mired the French economy in low growth and double-digit unemployment even as Britain and Germany outperformed. At 39 years old , he would be the youngest French president ; even his marriage to a woman 25 years his senior makes him appear refreshingly unconventional to some voters.

Mr. Macron’s challenge, if he wins, will be to provide solutions for those who have rallied around Ms. Le Pen, as much as for his own voters. He cannot cater to religious or racial resentments, but must be effective in fighting terrorism; he will have to increase employment through reforms of the labor code, not by protectionism or restriction of immigration. This will be particularly difficult if his new party fails to win a substantial place in Parliament in legislative elections next month. Yet failure would likely make Ms. Le Pen or another extremist the favorite in the next election.

In that sense, a Macron victory would offer the current French establishment, along with the European Union, a last chance. The centrist center appears to be holding on the continent following Britain’s E.U. exit vote and the U.S. presidential election. The defeat of Ms. Le Pen may help constrain the more radical impulses of President Trump, who has hinted at his sympathy for her. But unless the new government in France, and one to be elected later this year in Germany, can mitigate the negative effects of globalization and make E.U. institutions more democratic and accountable, the reprieve may be short-lived.