Spain’s governing Socialist party emerged on Sunday as the winner of regional elections in Catalonia that had been widely seen as a litmus test for Prime Minister Pedro Sánchez’s polarizing amnesty measure for separatists.

The Socialists are celebrating what they claim is a momentous victory, though they did not clinch enough seats to govern on their own. They most likely face weeks of bargaining, and possibly a repeat election if no agreement is reached. But for the first time in over a decade, they may be able to form a regional government led by an anti-independence party.

Addressing supporters late Sunday night at Socialist headquarters in Barcelona, the party’s Catalan leader, Salvador Illa, declared: “For the first time in 45 years, we have won the elections in Catalonia, in terms of both seats and votes. The Catalans have decided to open a new era.”

Still, Mr. Illa, who has promised improvements in social services, education and drought management, will need 68 of the Catalan Parliament’s 135 seats to form a government. On Sunday, his party got only 42, meaning he will have to seek support from the pro-independence party Esquerra Republicana de Catalunya (Catalan Republican Left) and the left-wing Comuns.

“Winning does not mean governing,” Toni Rodon, a professor of political science at Pompeu Fabra University in Barcelona, said before the results were in. While Esquerra has supported Mr. Sánchez in the Spanish Parliament, he said, negotiations in Catalonia are not expected to be easy.

The Socialists’ main rival was the pro-independence Junts per Catalunya (Together for Catalonia), led by Carles Puigdemont, who campaigned from exile in France. Junts came a close second, but with 35 seats would not be able to form a government with other pro-independence parties, which performed badly.

The leader of Esquerra, Pere Aragonès, who is also the departing president of the Catalan government, called the snap election after failing to garner enough support to pass a regional budget. After winning only 20 seats on Sunday, his party now faces a reckoning.

On Sunday night, Mr. Aragonés attributed Esquerra’s poor results to the party’s policy of making agreements with the Socialists, which he said, “have not been valued by the citizens.” From now on, he said, “Esquerra will be in the opposition.”

It was a clear indication that he is not willing to negotiate with Mr. Illa, and without the support of Esquerra, Catalonia could be “looking at a new election in October,” Professor Rodon said.

According to Ignacio Lago, a professor of political science at Pompeu Fabra University, even if no agreement is reached and the elections need to be repeated, “for the first time in years, the pro-independence parties do not hold the majority.”

The issue of an amnesty for separatists has been divisive for years.

When Mr. Sánchez first rose to power in 2019, he said he would not drop pending legal action against Mr. Puigdemont or others accused of separatist activity.

But Mr. Sánchez reversed himself after Spain’s general election last July, when his only chance for a second term required acceding to the demands of Mr. Puigdemont’s party, which had become kingmaker overnight by winning seven parliamentary seats. Mr. Sánchez, who is known as a political survivor, brokered an amnesty deal with Junts, calling it the best way forward for peaceful coexistence in Catalonia.

The amnesty proposal was wildly unpopular in Spain. Two rival parties organized an immense demonstration against the deal last November in cities around the country, and other protests not officially supported by the parties surged for nights on end outside the Socialist headquarters in Madrid.

At one point, a larger-than-life effigy of Mr. Sánchez with a long Pinocchio-style nose was beaten to smithereens by a mob.

The amnesty bill has stalled in the upper house of the Spanish Parliament after being approved by its lower house in March. Legal challenges could also still delay the measure.

Isabel Díaz Ayuso, head of the Madrid regional government and a member of the center-right People’s Party, has called the amnesty “the most corrupt law of our democracy.”

Historically, support for Catalan independence was no greater than 20 percent, according to a report publishedby the Elcano Royal Institute, an international affairs research group based in Madrid. That changed in 2010, after the financial crisis in the eurozone and austerity policies forced on Spain by the European Union encouraged “populist messages of fiscal rebellion” in Catalonia, the report said. The British government’s decision in 2012 to allow an independence referendum in Scotland bolstered separatists in Spain.

Tensions in Catalonia came to a head in 2017, when the separatist government led by Mr. Puigdemont ignored Spanish courts and moved ahead with an illegal independence referendum. A declaration of independence followed, as did a crackdown on the separatists by the Spanish government, which fired the Catalan government and imposed direct control. Nine political leaders were jailed for crimes including sedition, while Mr. Puigdemont fled to France, narrowly avoiding arrest.

Successive Spanish leaders, including Mr. Sánchez in his first term, have tried and failed to have Mr. Puigdemont extradited.

In 2021, Mr. Sánchez’s administration took a more conciliatory approach to Mr. Puigdemont’s allies still in Spain, pardoning the nine in prison.

The key question today, according to Cristina Monge, a professor of political science and sociology at the University of Zaragoza, is whether “the spirit” of the Catalan independence movement remains alive.

The positive election results for the Socialists in Catalonia on Sunday would suggest that the prime minister’s high-risk gamble to grant amnesty has paid off, reducing separatist tensions in the region and helping to normalize Spanish-Catalan relations.

“We have turned the page on the independence movement of 2017,” Professor Lago said.

A study conducted by the regional government’s Center of Opinion Studies shows that a rising share of Catalans — 51.1 percent in February, compared with 44.1 percent in March 2019 — support remaining in Spain.

Independence is no longer “a top priority for many voters,” Professor Rodon said, adding that the shift may reflect a general disenchantment with pro-independence parties rather than waning interest in separatism.