Traditional allies, like trade unions, are furious about labor market changes that they say favor business. Old-guard politicians, right and left, are wary that his diminution of the Senate, along with proposed changes to the electoral law and the Constitution, would place too much power in the executive branch.

“He is destroying all the checks and balances,” said Renato Brunetta, a lawmaker in Italy’s lower house of Parliament with the opposition party Forza Italia. “Renzi is moving the institutional structure of the country in an authoritarian direction.”

For years, entrenched leaders dominated Italy’s center-left and followed a consensus-driven model that critics say left the party paralyzed and beholden to special interests. Since his days as mayor of Florence, Mr. Renzi has openly challenged the older political generation, describing himself as the “Demolition Man” who would toss the established leadership onto the scrap heap — a stand that resonated with young people chafing for opportunities.

“There is a huge conflict between generations,” said Tommaso Giuntella, 30, president of the Democratic Party’s branch in Rome. “Renzi knows it and started being the face of the new generation, the generation of the sons. He said, ‘We cannot permit the son to pay for the privileges of the father.’ ”

This generational theme underpins Mr. Renzi’s most concrete legislative achievement: a new labor law, known as the Jobs Act, that he argues will gradually bring fairness to Italy’s bifurcated system of permanent and temporary work contracts.

He clashed sharply with the public employee unions, arguing that they were defending an inequitable system that ghettoized young workers while older workers clung to protected, permanent contracts. Unions countered that his law stripped away important job protections and made it easier for employers to fire people.

“He has chosen to represent entrepreneurs and the employers, not the workers,” said Serena Sorrentino, a senior official with CGIL, the country’s largest trade union. “He has broken with the traditions of the Democratic left.”