ROME — For all its melodrama and theatrics, Italian politics is often defined by what does not happen. The Italian Parliament has been deadlocked on major changes for years, unable or unwilling to enact bills to overhaul the political system or unshackle the economy, which has barely grown in two decades.

That is why the rise of Matteo Renzi, the charismatic young mayor of Florence, has sent a jolt through the landscape.

Mr. Renzi, 38, on Sunday became the leader of the country’s biggest party, the center-left Democrats, winning a nationwide primary by an unexpectedly wide margin — nearly 70 percent of the 2.9 million primary voters supported him. The outpouring was interpreted by many analysts as an unequivocal mandate to shake things up.

“This is so momentous,” said Franco Pavoncello, a political analyst and president of John Cabot University in Rome. “There has been a very strong resistance on the left to produce change.”