Emphatic, pugnacious and demanding: The style met the moment in the far-left leader Jean-Luc Mélenchon’s speech to a fired-up crowd of thousands celebrating victory in Sunday’s French legislative elections.

Standing before supporters in the working-class 20th arrondissement of Paris, Mr. Mélenchon addressed himself to President Emmanuel Macron, and not politely. “The president should either resign or name one of us prime minister,” he declared.

Other leftist leaders have said that there should be “discussions” about the future of the country. Not this one. The crowd on Sunday roared.

Mr. Mélenchon’s tone and hard-line stance have given him a devoted, youthful following — the only leftist leader with one — and made him both adored and hated, marginalized and central in French politics. More French have a negative opinion of him, 73 percent, than they do of Marine Le Pen, the leader of the far-right National Rally. But he also attracts large crowds who hang on his every word, as they did on Sunday.

Now he is necessarily at the center of the discussion of what might lie ahead for France: his brand of leftism or the milder form represented by his critics within the winning leftist coalition, the New Popular Front. His party, France Unbowed, won the most seats in Parliament, 75, in the coalition.