Ms. Le Pen is a populist agitator who, in the style of former President Donald J. Trump, scorns European Union “globalists,” criticizes NATO and views President Vladimir V. Putin of Russia as an ally.

Her victory could complicate Mr. Biden’s effort to isolate Russia and aid Ukraine. But the very real prospect of a nationalist leading France is also a reminder that the recent period of U.S.-European solidarity on political and security issues like Russia and democracy may be fragile. Poland and Hungary, both NATO members, have taken authoritarian turns. And Germany’s surprisingly strong response to Russia’s invasion of Ukraine is already drawing domestic criticism.

“To have a right-wing government come to power in France would be a political earthquake,” said Charles A. Kupchan, a professor at Georgetown who was the Europe director of the National Security Council during the Obama administration. “It would send a troubling signal about the overall political health of the Western world.”