Polls suggest that the National Front, renamed earlier this year as the Rassemblement National, might lose a significant number of votes in European elections in May if the Yellow Vests put forward their own candidates.

The anger coursing through French streets, which is now about much more than just the cost of driving, has created a highly combustible movement that stands against all political parties, including Ms. Le Pen’s, and threatens a plague on all their houses.

“We need to start again from zero,” said Stephane Bouillez, a long-distance truck driver who joined protesters in Hénin-Beaumont on Saturday. He said he could now barely feed his family because of a severe back problem that left him unable to work. To make ends meet, he set up a small company that sells army surplus clothing but quickly found that taxes and other costs ate up all its revenue and left him burdened with debts.

Last year, Mr. Bouillez said, he voted for Ms. Le Pen, and he said he thinks that Mayor Briois has done a good job since he and his far-right colleagues took over City Hall after local elections in 2014. The National Front now controls 29 of 35 council seats in a town that had been governed for decades by socialists.

But after taking part in weeks of demonstrations and freewheeling debate at protest camps, Mr. Bouillez decided it was time to break with all of France’s feuding political tribes. He scrawled that sentiment on the fluorescent yellow hazard vest that has been adopted as a symbol of the revolt: “France wake up! Stop being sheep.”

Whether the Yellow Vests will maintain their momentum and succeed in reshaping France is far from certain.