U.S. officials are anxiously watching the French presidential election, aware that the outcome of the vote on Sunday could scramble President Biden’s relations with Europe and reveal dangerous fissures in Western democracy.

President Emmanuel Macron of France has been a crucial partner as Mr. Biden has rebuilt relations with Europe, promoted democracy and forged a coalition in response to Russia’s invasion of Ukraine. But Mr. Macron is in a tight contest with Marine La Pen, a far-right challenger.

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.

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