ROME -- Nationwide primaries to choose the candidate who will lead Italy's center-left Democratic Party in elections next spring have ended without a clear winner, setting the stage for a run-off on Sunday between a seasoned party stalwart and a young upstart who has threatened to shake up Italian politics.
The party secretary, Pier Luigi Bersani, the front-runner, with 44.9 percent, will face Matteo Renzi, 37, the mayor of Florence, who campaigned to renew Italian politics starting with his own party. Mr. Renzi's 35.5 percent of the vote amounted to a significant challenge to the party's leadership, which had backed Mr. Bersani en masse.
The primary vote, held on Sunday, had been closely watched in Italy because, as things stand, the center-left is widely expected to win Italy's general elections next year, and the leader of the left has a strong chance of becoming Italy's next prime minister.
Through an effective grassroots campaign, Mr. Renzi tapped into the discontent of Italians with their political class and made a direct appeal to the young, a segment of society that continues to struggle to make strides in Italy.
But he alienated a chunk of the Democratic Party by seeking to broaden his appeal to disillusioned voters who once supported Silvio Berlusconi, the conservative former prime minister.
On Monday, Mr. Renzi vowed that the fight for the leadership of the coalition was not over and that voters were being asked to decide between two visions of the center left.
"It's a clear choice," he said. "Those who believe that the political class has done well in the past can vote Bersani and those who want change and innovation can vote for us."
The outcome of the runoff will lend a glimmer of clarity to Italy's garbled political arena, whose instability pushed the country to the brink of financial collapse a year ago. Since then, Italy has been governed by a technocratic government led by Mario Monti, whose own political future has increasingly become a matter of national interest.
Some centrist parties and a nascent political movement headed by the chairman of Ferrari, Luca Cordero di Montezemolo, have anointed Mr. Monti as their ideological leader, capitalizing on his international reputation and promising to continue to pursue his reformist agenda, which he has only partly succeeded in pushing through Parliament.
Mr. Monti has not ruled out playing a role in the country's future after general elections next year, and said on television Sunday night that he would reflect on how he might contribute.
Commentators say that a new Monti-led government would be a relief for international markets, which remain jittery because the likelihood of a strong party emerging from the elections remains slim. Italy's modern political history is filled with fragile, short-lived coalition governments.
One brief exception was Mr. Berlusconi's conservative alliance, which now trails third in polls and has struggled to find its footing since the former prime minister left government last November. Mr. Berlusconi has hinted again that he would run for office, with a new party.
"We are evaluating the situation," which will depend on the outcome of the center-left primaries, he said.
This article originally appeared in The New York Times.