After months of trying to move the political needle in favor of Marine Le Pen in the French presidential election, American far-right activists on Saturday threw their weight behind a hacking attack against her rival, Emmanuel Macron, hoping to cast doubt on an election that is pivotal to France and the wider world.

The efforts were the culmination of an extended campaign against Mr. Macron after his candidacy began to gain steam this year, with digital activists in the United States and elsewhere sharing tactics, tips and tricks across the English- and French-speaking parts of the internet.

It is unclear whether the leaked documents, which some experts say may be connected to hackers linked to Russia, will affect the outcome of the election on Sunday between Ms. Le Pen, the far-right candidate from the National Front, and Mr. Macron, an independent centrist. But the role of American far-right groups in promoting the breach online highlights their growing resolve to spread extremist messages beyond the United States.

“It’s the anti-globalists trying to go global,” said Ben Nimmo, a senior fellow of the digital forensics research lab at the Atlantic Council, a think tank, who has studied the far right’s recent efforts against Mr. Macron and others in France. “There’s a feeling of trying to export the revolution.”