Stefano Bonaccini, president of the Emilia-Romagna region in northern Italy, won just 46% of the vote after being projected to win easily. His support was mainly concentrated in the conservative southern regions of the country.
Schlein will now lead the PD in opposing the government of Prime Minister Giorgia Meloni, whose Brothers of Italy party has neofascist roots and who has been condemned for pushing discriminatory education policies and penalizing humanitarian groups that rescue migrants in the Mediterranean Sea.
The new PD leader addressed the latter issue on Sunday, as her victory came the same day dozens of refugees, including 20 children, died when their overcrowded boat capsized in the sea—days after Italy's parliament passed a new law imposing restrictions on rescue boats, making it more difficult for charities to save asylum-seekers.
The refugees' deaths weigh "on the conscience of those who only weeks ago approved a decree whose only goal is to hinder rescues at sea," said Schlein on Sunday, calling for migrants to be permitted to legally apply for entry into all European nations and for the E.U.'s government to strengthen search-and-rescue efforts in the Mediterranean.
Schlein promised that under her leadership, the PD "will be a problem" for Meloni's government.
"She's a force to be reckoned with," said journalist Andrea Carlo. "I imagine Meloni & Co. won't be sleeping too well tonight."
The 37-year-old former member of European Parliament has been called "Italy's AOC" by some news outlets—referring to U.S. Rep. Alexandria Ocasio-Cortez (D-N.Y.)—for her support for a minimum wage law and a Green New Deal to create jobs and help the country drastically reduce its fossil fuel emissions.
Last year, she announced her campaign to lead the PD as one that would be "progressive, environmentalist, and feminist."
Schlein's victory represents "a genuine moment of hope in the fight against the far right in Italy, and across Europe," said socialist activist Michael Chessum.
At one point Schlein was polling 18 points behind Bonaccini. Her surprising margin of victory was secured largely thanks to the support of women and young voters, according to the Associated Press.
"The Democratic Party is alive and ready to stand up," said Schlein. "We did it, together we made a small big revolution, even this time they didn't see us coming."
Schlein's victory came as trade unions across Italy demanded better safety protections and job security for port workers, holding a nationwide maritime port strike Saturday. In December, the PD and unions organized street protests over Meloni's proposed budget, which they said targeted the poor by cutting the country's "citizen's wage" for unemployed people and not addressing rising costs of essentials.
"We will put the battle against every type of inequality and precariousness center-stage," said Schlein on Sunday.
