Ms. Le Pen’s social media team has fought a guerrilla-style war to spread its message online, including a dedicated group that shares videos and photos online that attack her political foes. A loose network of Facebook and Twitter users has similarly backed her campaign while disparaging Emmanuel Macron, Ms. Le Pen’s opponent and the front-runner to be France’s next president. Many of these social media messages have been shared by the supporters of more traditional politicians, including those of François Fillon, a right-wing candidate who finished third in last month’s first-round election.

While muted, American-style fake news has also made an appearance.

Ahead of last month’s vote, for instance, a fake news site masquerading as Le Soir, a Belgian newspaper, tried to spread rumors that Saudi Arabia was financing Mr. Macron’s campaign. Marion Marechal-Le Pen, a niece of Ms. Le Pen, posted the piece on Twitter before quickly removing the link after local media outlets debunked the claim.

Still, for many in France, such outright fake news stories have been met merely with Gallic shrugs. And the digital tactics of international campaigners have been even less effective.