ROME — Matteo Renzi became Italy’s youngest premier on Saturday, promising a new era of stable government after using old-school politicking to engineer the ouster of fellow Democrat Enrico Letta, whom he deemed too timid to get the nation back to work.

Chief challenge for his broad coalition is the ailing economy, only just beginning to show signs of rebounding after several years of stagnation and with youth unemployment hovering around 40 percent. The unabashedly ambitious Renzi, 39, quit his post as Florence mayor to take up his first national government job.

He has vowed to push electoral reforms through Parliament.