BELGRADE, Serbia-After Serbia's weekend national elections, all eyes will once again be on Vojislav Kostunica.

The outgoing prime minister is already reveling in his role as the kingmaker with the keys to Serbia's future course: A steady journey toward EU membership or a return to a nationalist past with Russia as a guide and protector.

There is little doubt which side he is likely to take.

Although Kostunica was the man who led the popular revolt that toppled strongman Slobodan Milosevic in 2000, he has since become an outspoken nationalist who has become a major obstacle to integration with the European Union.

Polls released ahead of Sunday's vote show the ultranationalist Radicals with 34 percent support, against 33 percent for a pro-Western coalition led by President Boris Tadic. Kostunica's populist bloc is third with 12 percent, likely giving him a decisive voice in forming the next government.

The 64-year-old law professor has based his campaign on Serbian outrage over Western support for Kosovo independence. Publicly he has remained noncommittal on which side to support, but his coalition deputy, Velimir Ilic, is clear on the question of allegiances.

"We will never again form a government with traitors like Tadic," Ilic said. "Radicals are the best option, why not?"

Tomislav Nikolic, the head of the far-right Radicals, said: "There are only two postelection scenarios, our coalition with Kostunica, or another election if that coalition is not formed."

"Our views on Serbia's future are almost identical," Nikolic said of Kostunica in an interview with The Associated Press. "We would make a strong government."

Serbs consider Kosovo their traditional heartland, and the West's backing for its statehood has driven many of Serbia's 6.7 million voters into the arms of the nationalists.

The nationalists are also capitalizing on disenchantment with 30 percent unemployment, rising prices and corruption.

Both Kostunica and Nikolic have said Serbia should shelve its proclaimed goal of joining the EU and concentrate instead on establishing close political and economic ties with Russia, which has supported Serbia's claim on Kosovo.

The International Crisis Group, a Brussels-based think tank with a long history in the Balkans, said: "Kosovo's independence declaration ... sent shock waves through Serbia's politics and society, polarizing the former in a manner not seen since the Milosevic era."

Hoping to help Tadic counter the Kosovo backlash, the European Union signed a pre-entry aid and trade deal with Serbia last week, aiming to show voters the benefits of casting a pro-Western vote.

The nationalists have labeled Tadic "a traitor" for endorsing the deal, which Kostunica and Nikolic say is "a signature for an independent Kosovo."

"It was ordered by Washington, then obeyed in Brussels and then someone in Belgrade agreed to it," Kostunica said, adding: "Serbia needs a nationally responsible government that will tell the world that Kosovo is Serbia and will always remain so."

Tadic said Kostunica is "brutally lying to the people by saying Serbia's pro-European path is a way to lose Kosovo. The May 11 election will be a referendum which will decide whether Serbia will join the European Union, or return to its isolation and general despair."

Tadic pledged that Kostunica, who took over Serbia's presidency after Milosevic's ouster, and held two premierships posts since 2000, "will never again be a prime minister."

But the presidency gives Tadic little power to block the ruling coalition's choice for premier.

Some fear that Serbia will sink into deep isolation if Kostunica forms a government with the Radicals.

"They want to drape us in black, build a Berlin Wall between us and the European Union, all under the pretext of fighting for Kosovo," said Vuk Draskovic, a former foreign minister and a staunch opponent of Milosevic's regime in the 1990s.

"It would be a true nightmare," he says, "to wake up on May 12 and see that those I have fought against for 20 years have returned to power."