Ms. Le Pen’s tactics on Wednesday, eschewing any kind of detailed exposition of policies and instead relying on epithet-slinging — Mr. Macron was “the privileged child of the system and the elites,” and the “representative of subjugated France” — would have been familiar to anyone attending her rallies across France this election season. Her supporters roar at these verbal sallies.

But such language is not normally part of mainstream political discourse in France. And that fact set up the collision of Wednesday night, and the tone of dismay and shock in the commentaries.

The second-round presidential debate has become almost a sacred ritual in French politics. Fifteen years ago, Jacques Chirac, the former president, refused to dignify Ms. Le Pen’s father in a debate when he unexpectedly made the second round. That Ms. Le Pen was not given that treatment in 2017, commentators suggested, meant that she had been accepted as a legitimate partner in the democratic process.

But on Thursday, French news media and academic commentators suggested she had violated that trust by her “violence,” as many put it. “Maybe she wanted to reassure her electorate,” Marc Lazar, a historian, said in an interview, “or maybe she was just showing her true nature.”

“She has wanted to show that she has ‘undemonized’ the party,” Mr. Lazar continued, referring to the effort Ms. Le Pen has undertaken to distance the National Front from the hate-filled declarations of her father. “But in the end, she just proved that she is her father’s daughter. I think there were a lot of people who were surprised, because they thought she had really changed.”

Even veteran Front-watchers were taken aback by Ms. Le Pen’s actions on Wednesday night. “It was transformed into a fight, not a debate,” said Valérie Igounet, the leading historian of the National Front.

Numerous political figures said the debate had made a big voter turnout for Mr. Macron all the more urgent. It was not expected to come from the far left, which continues to evince extreme hostility to Mr. Macron, seeing him as the hated representative of capitalism and finance — precisely Ms. Le Pen’s depiction of him.