Serbia's two presidential challengers have clashed in a televised debate ahead of a run-off vote on Sunday that pollsters say is too close to call.
Mr Tadic says EU membership will mean a better life for Serbs
Incumbent President Boris Tadic said Serbia was facing "a stark choice: the European Union or isolation".
His nationalist rival Tomislav Nikolic, who narrowly won the first round, said he preferred close ties with Russia.
The poll is seen as key for Serbia's future as the province of Kosovo prepares to declare independence.
Kosovo has been run by the UN since 1999, when a US-led Nato bombing campaign drove out Serb forces accused of a brutal crackdown on ethnic Albanian separatism.
The US and most of the EU members back the independence plan, while Serbia and Russia are strongly opposed to it.
"We are at the crossroads," Mr Tadic, who heads the Democratic Party of Serbia, said during the debate on Wednesday night.
Mr Nikolic lost to Mr Tadic in the 2004 presidential run-off
Mr Tadic urged voters to choose the path of integration with the EU, warning that the alternative was isolation of Serbia in Europe.
For his part, Radical Party leader Tomislav Nikolic said that Serbia had "two roads, one which is open toward the Russian Federation, and the other which is thorny toward the European Union".
"Russia is our friend who will prevent the United States and the European Union from taking Kosovo away from us," he said.
Like Mr Nikolic, President Tadic strongly opposes the idea of an independent Kosovo.
But during the election campaign Mr Nikolic has projected a more hardline stance on ties with the EU if the bloc recognises Kosovo's independence.
In a separate development, Serbian Prime Minister Vojislav Kostunica on Wednesday refused to endorse Mr Tadic in the run-off.
"It's up to the voters to decide whom they will support," Mr Kostunica said.
He had been expected to back his coalition partner, and without his support pollsters say the race is too close to call.
However, the prime minister stressed that he did not back Mr Nikolic either.
Ethnic Albanian leaders in Kosovo say a declaration of independence is "an issue of days".
"There are some procedures we need to respect and some consultations," Kosovo's Prime Minister Hashim Thaci said last week after meeting EU foreign policy chief Javier Solana in Brussels.
Kosovo is still technically a southern province of Serbia.
EU states have asked Kosovo's leadership to wait until an EU civilian mission can take over from the UN.
A police and judicial staff of about 1,800 is being prepared to work in tandem with the existing Nato-led force.