BELGRADE, Serbia — Serbians voted Sunday in a presidential runoff election that pits pro-European Union Boris Tadic against nationalists Tomislav Nikolic who wants closer ties with Russia and is threatening protests if he loses because of alleged ballot rigging.

The vote is key for Serbia's plans to become an EU member, after being an isolated pariah state under late autocrat Slobodan Milosevic in the 1990s. It also will determine whether Serbia continues to reconcile with its neighbors and wartime foes, including the former province of Kosovo, which declared independence in 2008.

Tadic, who is seeking a third term, was slightly ahead of Nikolic in the first round of voting on May 6, while Nikolic's Serbian Progressive Party won the most votes for parliament, but is likely to stay without power because Tadic's Democrats have agreed to form the next government with the third-placed Socialists.

The nationalists have accused the Democrats of rigging the general vote, including the first-round presidential ballot — the charge that was rejected by authorities, but is fueling fears of possible post-election violence.

Tadic, who championed Serbia's bid to become an EU member, was leading the pre-runoff polls, but the pro-democratic voters are known to be less enthusiastic than the nationalists when it comes to casting their ballots.

Tadic has built his presidential bid around his pro-Western policies, but his biggest problem remains the economic downturn and corruption within the ruling elite. Faced with the global financial crisis, which has slowed down much needed foreign investments, Tadic's government has seen massive job losses and plummeting living standards.

Nikolic, who narrowly lost two earlier presidential votes against Tadic, claims to have shifted from being staunchly anti-Western to pro-EU, and says that he wants Serbia both "in the West and the East." Nikolic has close ties with Russia and has even in the past envisaged Serbia as a Russian province.

Nikolic's populism has struck a chord with voters, who warmed to his criticism of widespread social injustice and corruption in Serbia and promises of jobs, financial security and billions of dollars in foreign investment.

Tadic has also overseen a more conciliatory stance toward Serbia's neighbors and war foes in the 1990s, including Kosovo, the former province that is considered by nationalists as the cradle of Serbia's state and religion.

Nikolic has said he would abandon the EU plan if it means that Serbia must give up its claim on Kosovo, and he has forged an alliance with the staunchly anti-EU party of former Prime Minister Vojislav Kostunica.