The Serbian party founded by Slobodan Milosevic, the former president, has emerged as potential kingmakers after general elections in which neither the liberal 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.
The opposition right-wing populist Serbian Progressive Party (SNS) took 73 seats in the 250-member assembly, ahead of the Democrat Party (DS), which took 67 seats. Neither party has enough to govern on its own. Dacic's Socialists won 44 seats.
"We have risen from the ashes," a triumphant Dacic said.
Dacci said he would seek to be prime minister in any future government, and left the door open for negotiations with both the DS and SNS.
"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.
In presidential elections, also held on Sunday, a near-complete official vote count released on Monday confirmed that a runoff will be held on May 20 between the DS's Boris Tadic, who won 25.3 per cent of the vote, and the SNS's Tomislav Nikolic, who had 24.9 per cent.
Analysts predicted that the Socialists could wait for the outcome of the presidential runoff before they decide which way to turn this time.
The Socialists were allied with the DS 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 country's conflicts and pushed the country into international isolation.
In the run-up to the elections, Dacic toughened his positions while calling for social justice. His defiant, anti-Western campaigning evoked the style of his former patron Milosevic.
Nebojsa Spaic, editor in chief of NIN weekly, told the AP news agency 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 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.
Nikolic, a 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.