The Socialist leader, Ivica Dacic, said the nationalists are offering him to become the prime minister in the next government, while in a Cabinet led by the pro-EU Democrats he and his party officials would only be ministers, like they were in the outgoing government.
"Why would we be in someone's government if we could lead it?" said Dacic, who heads the leftist party formed by late Serbian autocrat Slobodan Milosevic in the 1990s.
Neither the Democrats nor the nationalist Progressive Party won enough votes in Serbia's parliamentary election in May to rule alone, and they are trying to lure Dacic's Socialists to form a coalition government that the EU hopes would continue with the social and economic reforms that would lead to membership in the bloc.
But the formation of a so-called "red-black" coalition of the Socialists and the nationalists would mark the return of Milosevic's allies to power in Serbia for the first time since his ouster in a popular uprising in 2000. Six years later, Milosevic died during his war crimes trial at a U.N. tribunal.
Such a coalition also would represent another blow to the Democratic party leader, Boris Tadic, who was defeated in presidential election in May by nationalist Tomislav Nikolic.
Tadic's Democrats said in a statement that Dadic's Socialists should say what has been stalling the tentative government deal the two parties had reached soon after the parliamentary elections to form Serbia's next government.
But Dacic said that Tadic's loss in the presidential vote to Nikolic "changed the political scene" in Serbia and that it would be better for his party to make a deal with the nationalists. He said his party leadership will make a decision soon.
Dacic, who has made two trips to Russia since the elections, said earlier that the Kremlin was pressuring him to form the coalition government with the nationalists. Western officials had hoped that Tadic would head the next Serbian government, partially isolating Nikolic's the pro-Russian influence.
Although Nikolic has claimed to have shifted from being staunchly anti-Western to pro-EU, the formation of the nationalist government would complicate Serbia's proclaimed EU bid and increase Russian influence in the Balkans.