PARIS — He left the corruption scandal-plagued Marine Le Pen of the far-right National Front speechless. He made the nepotism-tarred center-right candidate, François Fillon, grin frigidly, murmuring about a lawsuit. And from the immaculately groomed television anchors he drew only condescending fixed smiles.

It took barely over a minute for Philippe Poutou, a balding and unkempt Ford factory mechanic from Bordeaux running as a fringe candidate in France’s presidential election, to puncture the mutually protective world of the race’s mainstream. Afterward television commentators tut-tutted about his “lack of respect.” That was exactly Mr. Poutou’s point.

So effective was his brutal anti-corruption language that two days after a marathon four-hour debate among all 11 French presidential candidates, the rumpled no-hope candidate of the New Anticapitalist Party was being hailed in some news media here as its unquestioned winner. Mr. Poutou, 50, had instantly become a kind of folk hero, one expressing in unvarnished form what many Frenchmen and women are thinking: The political class, encased in its privileges, is demonstrably corrupt.

The corruption scandals, widely aired in the print press, have been mostly taboo beneath a well-maintained facade of respectability in these debates.

But Mr. Poutou did not plan to ignore them. Instead of a suit, he wore a white henley T-shirt and had barely shaved. He refused to pose for the group photo at the beginning.

“It’s not because I’m not wearing a tie that you can cut me off,” he said sharply to one of the debate hosts, who was trying to curb him. Running from a far-left party that got 1 percent of the vote in the last election, Mr. Poutou had little to lose.

There was going to be trouble.

It came like a Scud missile, as Le Monde put it.

Mr. Poutou turned to Ms. Le Pen, whose surging National Front appears on the verge of a historic breakthrough, all but guaranteed a place in May’s presidential runoff against the other probable finalist, the former economy minister Emmanuel Macron.

At the same time, her party is mired in twin corruption scandals, one involving fake jobs at the European Parliament, the other fake campaign-expense billing reimbursed by the state. Close associates have been charged.

Ms. Le Pen, a supposed “outsider” candidate from a populist party, has refused to answer a police summons, invoking “parliamentary immunity.”

That angered Mr. Poutou. He didn’t bother with honorifics or first names — another sore point for the offended television commentators.

“And then we’ve got Le Pen, next to me, Le Pen, dipping into the public purse,” Mr. Poutou said. “It’s not here, it’s Europe, and now for somebody who is anti-European, the worst is that the National Front, which calls itself anti-system, it doesn’t give a damn. It protects itself thanks to the laws of the system, thanks to parliamentary immunity, and so refuses to answer a police summons.”

Then he scored what was acknowledged as the evening’s bull’s-eye, aimed squarely at Ms. Le Pen: “When we workers are summoned by the police, we don’t have worker’s immunity. Sorry about that. There it is. Here we go,” Mr. Poutou said.

He didn’t look as if he was joking. The studio audience burst into applause. Ms. Le Pen, visibly flustered, was left uncharacteristically silent. She had been attacked by a factory worker for being a privileged child of the system — a nuclear hit against a rabble-rousing candidate whose rallies are always framed by banners with the words “In the name of the people.”

“The blow,” Le Monde commented, “was sharp.”

Editorial writers across France hailed Mr. Poutou, and his sally lit up social media.

“One came out of it the big winner: Philippe Poutou,” L’Est Républicain, based in Nancy, wrote. “In shattering the system’s formalities, Philippe Poutou gave form, using simple words, to a widespread sentiment.”

Mr. Poutou had “shattered the screen,” Le Courrier Picard wrote. “It was like the eruption of a certain reality into the weighty and rigid universe of the political-media sphere.”

“The devil Poutou leaped from his cage to attack Fillon,” La République des Pyrénées said, “and then Marine Le Pen. He scored the winning hit. This is what will be remembered.”

Mr. Fillon, the candidate of the Républicains, and at one time the race’s front-runner, was hardly spared by Mr. Poutou. He has dropped steadily in the polls since being put under investigation, and now criminally charged, over accusations that he used public money to pay his wife for a phony job as a parliamentary assistant.

Revelations that a political fixer recently bought him two fine suits for over $13,000 at one of Paris’s fanciest tailors have not helped. Mr. Fillon’s image of probity has been shattered, and his platform of austerity for France is the object of ridicule.

“Fillon. Right. He’s sitting across the room from me,” Mr. Poutou began. His target shifted uneasily in his seat, a tight smile on his face. “Nothing but revelations,” Mr. Poutou continued. “The more one digs, the more one smells corruption and cheating, and then, it’s these fine folk who explain to us that we need rigor, austerity, and all the while dipping into the public purse.”

Le Monde heard Mr. Fillon mutter furiously, “I’m going to sock you with a damn lawsuit, you.”

Afterward, the television commentators in Paris sniffed. “I don’t think Philippe Poutou deserves any honors,” said Anna Cabana, an analyst on BFMTV. “He acted very disrespectfully.”

But that was not the verdict on social media. And Mr. Fillon is now in danger of losing even his third-place perch in the first round of voting April 23 to another insurgent far-left candidate, the veteran Jean-Luc Mélenchon, whose relatively unbridled style and populist fervor have caught on.

Mr. Poutou was enjoying his new celebrity Thursday.

“It’s a bit surprising. We’re not used to it,” Mr. Poutou, a mechanic in the maintenance and repair department of a Ford factory, said in a brief phone interview, dashing between appointments. “In the streets, it’s very striking. People are saying bravo, thanks so much. We realize it’s touched a lot of people. It’s what people are thinking, but these things are almost never said to politicians’ faces.”

ADAM NOSSITER