The leader of a far-left movement who won nearly 20 percent of the vote in the first round of France’s presidential election, Jean-Luc Mélenchon, told his seven million voters in a YouTube address on Friday that he would not tell them how to vote in the final-round run-off next weekend.

As for himself, Mélenchon said that he would cast a ballot, and that it would not be for Marine Le Pen, the candidate of the far-right National Front, who courted his voters in a video of her own on Friday. But Mélenchon also refused to say, like the leaders of other parties across the political spectrum — and celebrities including the French soccer legend Zinedine Zidane — that he would vote for Le Pen’s centrist rival, the former banker Emmanuel Macron, to stop the far-right from gaining power.

Instead, Mélenchon predicted that forcing France to choose between a candidate of “the extreme right” and one of “extreme finance” would led to a political crisis, and left open the possibility that he would submit a blank ballot, a form of protest vote permitted under French electoral law. (Mélenchon’s platform included provisions for voting to be made mandatory, and for blank ballots to be recognized under law.)

The appeal for unity, to construct a barrage, or dam, against the rising tide of the far-right, Mélenchon said, was, in fact, a disguised attempt to force voters like him, who profoundly disagree with Macron’s economic policies, to endorse his project.

Amid fears that widespread abstention and protest votes for neither candidate could lower the threshold for Le Pen to win with 50 percent of the valid votes cast, Mélenchon’s refusal to join the sort of united front against Le Pen that led to her father’s defeat in 2002 caused anxiety to spike.

Those fears were reflected on the front page of Saturday’s Libération, the Paris daily, which tried to shock left-wing voters into casting votes for Macron by imagining a Le Pen victory with depressed turnout and a high number of blank protest votes.

They also inspired the Charlie Hebdo cartoonist Corinne Rey, who uses the pen name Coco, to predict that voters who refused to vote for Macron for any reason would be sorry if they lowered the bar for a victorious Le Pen.

“Macron is not my candidate,” the cartoonist explained on Twitter, but added a hashtag calling for a united front against the National Front.

Mélenchon’s YouTube address was released hours after Le Pen posted an appeal to his voters on social networks, in which she tried to turn the concept of a united front against her extreme nationalist party on its head, calling on voters of the far-left to unite with her to block Macron, a former economy minister who is liberal on social issues but neoliberal on economics.

Raquel Garrido, a spokeswoman for Mélenchon, called Le Pen’s appeal to his supporters “an insult to their intelligence.”

Another Mélenchon spokesman, Alexis Corbiere, icily noted Le Pen’s “despicable video” had garnered only a few thousand views all afternoon, in contrast to the message from his party leader, which surpassed 110,000 views in an hour.

At several points in the 32-minute message to his supporters, Mélenchon expressed indignation at the idea that he, or any large number of his voters, could ever support Le Pen. “Is there a single person who doubts that I will not vote National Front?” he asked rhetorically. “My opinion is displayed on all my clothes for five years,” he said, pointing to a small red triangle pin on his jacket — a symbol of communists deported to Nazi concentration camps during the French Vichy regime Le Pen and her party apologizes for.

“And besides, of the 7 million people who voted for me,” Mélenchon added, “I am almost certain that only a small fraction part will vote National Front.”

“I am not a guru, I am not a guide,” he said, standing by his refusal to endorse Macron. “I am a political leader who tries to shed light on the path,” he added, perhaps hinting that he wants to keep his new party intact ahead of the legislative elections that directly follow the presidential vote.

Top Photo: A hologram of Jean-Luc Mélenchon addressing supporters in Montpellier, France earlier this month, before he was eliminated in the first roung of France’s presidential election.