Tadic holds that the way to contain Kosovo is through closer ties with the West.
He opposes independence for the province, seen by Serbs as their historic heartland, but favours signing a first-level agreement with the European Union, even if the bloc takes over Kosovo's supervision as a prelude to recognising the territory.
Nikolic, whose Serbian Radical Party (SRS) supported the policies of late ruler Slobodan Milosevic, puts his money on Russia.
Tadic, speaking about Nikolic winning the presidential election, said: "There is a risk of the return of the old guard that led Serbia into conflicts and isolation during the 1990s and led people into turmoil and uncertainty.
"But I am certain the people will not allow this, they will vote for stability and economic progress, for a better life for their families."
Nikolic, who has toned down his rhetoric to appeal to moderates as well as the one third of Serbs who live just over the poverty line, has rejected accusations of isolationism and war-mongering.
|"I have not said goodbye to the West. I said we could be with you, and that we can sign whatever you want, but do not touch Serbia"|
Tomislav Nikolic, Serbian Radical Party presidential candidate
At a campaign rally this week, he said: "I have not said goodbye to the West. I said we could be with you, and that we can sign whatever you want, but do not touch Serbia.
"Serbia has its borders."
Kosovo has been a United Nations protectorate since 1999, when Nato expelled Serb troops accused of atrocities against ethnic Albanians there while fighting a guerrilla war.
Indications by the US and most EU member states that they will recognise Kosovo as independent within months have irked Serbs who feel the country has paid enough for its role in the wars of the 1990s.
Serbian resentment at Western support for Kosovo could win Nikolic's party more votes.
Polls suggest Nikolic will win 33 per cent of the first-round vote, with Tadic around 30 per cent.
Three other candidates are seen winning about six per cent each. Only one of those, Cedomir Jovanovic, is prepared to tell Serbs that Kosovo is lost and that the country should move on.
However, half of the 6.5 million electorate may stay home, reflecting disappointment with democratic politics seven years after the fall of Milosevic.
Apart from Kosovo, the country also faces the challenge of fulfilling its obligations to the war crimes tribunal in The Hague by arresting ethnic Serb war crime suspects from the Yugoslav wars.
The EU has said Belgrade will be welcome as an EU candidate once it has shown it can face up to its past by arresting the men accused of atrocities, who are believed to be hiding in Serbia with the help of hardliners.