The Serbian party founded by late strongman Slobodan Milosevic emerged on Monday as potential kingmakers, after general elections in which neither the pro-EU nor nationalist camps clinched clear victory.
Socialist Ivica Dacic's party doubled its tally in Sunday's ballot from the last elections, achieving its best result since Milosevic was ousted from power in a pro-democracy uprising in 2000.
"We have risen from the ashes," a triumphant Dacic said.
He said he will seek to be prime minister in any future government, and left the door open for negotiations with both the incumbent pro-EU Democrats and opposition right-wing populist Serbian Progressive Party.
"If we still don't know who will be Serbia's next president, I think we know who will be the prime minister," he declared confidently at a celebration late on Sunday.
A near-complete official vote count released Monday confirmed that a presidential runoff will be held on May 20 between pro-Western leader Boris Tadic, who won 25.3 percent of the vote, and nationalist Tomislav Nikolic, who had 24.9 percent.
In the parliamentary vote, the results showed Nikolic's Progressives winning 73 seats in the 250-member assembly, ahead of Tadic's Democrats, which took 67 seats. Neither party has enough to govern on its own. Dacic's Socialists won 44 seats.
The Socialists were allied with Tadic's Democrats in the previous Serbian government, supporting EU integration and reconciliation with former war foes in the Balkans. It was a major shift from the warmongering policies of Milosevic, who ignited the conflicts and pushed the country into international isolation.
In the run-up to the elections, Dacic toughened his stands while calling for social justice. His defiant, anti-Western campaigning evoked the style of his former patron Milosevic.
Analysts predicted that the Socialists could wait for the outcome of the presidential runoff before they decide which way to turn this time.
Nebojsa Spaic, editor in chief of NIN weekly, told The Associated Press that Dacic played on the populist card, which "obviously made him popular."
"Now, he is in a position that any future government depends on him, and I believe that he will go with the winner of the presidential elections," Spaic said.
Tadic, who advocates swift EU integration and reform, said that the presidential runoff will be crucial and "determine what Serbia will look like in the next five years."
He warned he "will not be blackmailed" by the Socialists in forming the next government.
"The battle will be fought between myself and Nikolic," Tadic said. "Our policies are substantially different, we have different values, we have different character."
Nikolic, a somber former cemetery manager who was allied with Milosevic in the 1990s, says he, too, supports EU integration, but also wants much closer ties with Serbia's traditional ally, Russia. He predicted he will win the runoff.
"Victory is within reach," Nikolic said. "We will have a new government and a new president."
Such a scenario would mark the first time that allies of Milosevic fully return to power since 2000. That would affect the pace of Serbia's EU-demanded economic and social reforms, and Serbia's reconciliation with its wartime foes, including the former province of Kosovo, which declared independence in 2008.
The reformist Democrats' popularity was threatened because of Serbia's economic problems and alleged corruption among the ruling elites. Faced with the global financial crisis, which slowed down much needed foreign investments, Tadic's government has seen major job losses and falling living standards.
Nikolic tried to get voter support by criticizing widespread social injustice and by promising jobs, financial security and billions of dollars in foreign investments if he and his party win the election.
Associated Press correspondent Dusan Stojanovic contributed.