By Matt Robinson
BELGRADE (Reuters) - Challenger Tomislav Nikolic may not win, but he can sure spoil the party as Boris Tadic bids for a new term as president of Serbia and the right to lead the country into talks on joining the European Union.
For the third time since 2004, reformist Tadic is poised to defeat Nikolic in Sunday's run-off, giving him five more years at the helm of Serbia as it slowly sheds the legacy of late strongman Slobodan Milosevic and the bloody collapse of Yugoslavia.
But an acrimonious row over alleged election fraud and talk of a "fight" to defend the vote has revived memories of the dark days of a divided Serbia under Milosevic and threatens to overshadow a Tadic victory.
Election authorities and foreign monitors found no evidence of the 500,000 votes Nikolic says were forged in parliamentary and first-round presidential elections on May 6.
But he's threatening to call supporters into the streets anyway, dumping on the steps of parliament sacks of ballot papers he says were stolen from him and his rightist Serbian Progressive Party, the country's biggest party.
"God only knows what happened," said pollster Marko Blagojevic of the Centre for Free Elections and Democracy (CESID). But, he said, "It could lead to a serious rise in tensions."
The row has brought back unhappy memories for many Serbs who marched in the streets against rigged elections under Milosevic, finally toppling him in October 2000 after a decade of war and isolation.
Tadic, 54, says his opponent, a former member of the ultranationalist Radical Party, is trying to undermine Serbian democracy and invent an excuse for his likely loss at the ballot box.
The election, Tadic told the Serbian daily Kurir on Wednesday, "will decide the future and whether it will be safe and peaceful, or we will enter a period of instability and crisis."
Nikolic's Progressives narrowly won the May 6 parliamentary election, but face being locked out of government by a renewed governing coalition announced last week between Tadic's Democratic Party and the third-placed Socialists once led by Milosevic.
Their uneasy alliance, in power since 2008, has a patchy record on reform but has kept the country edging towards the EU, arresting and extraditing Bosnian Serb war crimes fugitive Ratko Mladic and improving relations with Serbia's ex-Yugoslav neighbors.
Nikolic has tried to rebrand himself from ultranationalist to modern, pro-European conservative. His conversion marked a watershed for Serbia, with the country's main political forces converging around the common aim of EU membership.
But he has struggled to convince voters of the change, and critics say the fraud row looks a lot like the Nikolic of old, a former disciple of firebrand war crimes suspect Vojislav Seselj and partner in government with Milosevic when Serbia was bombed by NATO in 1999.
"If they don't drop this, and there's no sign the Progressives will drop this, then certainly doubt will hover over Tadic's government if his party forms the government, and most probably it will," said CESID's Blagojevic.
"That shadow of doubt will hang over his government regardless of whether or not the relevant institutions get to the bottom of the case."
Campaigning in open-neck shirt, sleeves rolled up, Tadic has nurtured a carefully-crafted image of a reliable, charming family-man, a safe pair of hands after years of turmoil and poverty.
Nikolic, a 60-year-old former cemetery manager, has jettisoned the symbolism of hard-line patriotism but is a stiff and uninspiring orator.
He was beaten by Tadic in the first round by less than one percentage point, but the incumbent has since won a flurry of endorsements from political allies. A poll by Ipsos Strategic Marketing after the first round saw Tadic winning by 58 percent to 42 percent.
Tadic's Democratic Party will need political stability if it is to clinch a date for the start of EU accession talks, possibly early next year. Neighboring Croatia will join the EU in July next year, driving home for many Serbs just how far they have fallen behind.
Under the constitution, the prime minister is more powerful than the president. A victory for Nikolic on Sunday would usher in a period of difficult "cohabitation" with a Democrat-led government, with the president able to block or hold up legislation.
As leader of the Democratic Party, Tadic has firmly guided policy over the past four years, with his party the dominant partner in the government and quiet technocrat Mirko Cvetkovic in the post of prime minister.
The Socialists, who doubled their number of seats in parliament to 44 in the May 6 election, say they want to lead the government, possibly with Milosevic's wartime spokesman Ivica Dacic as prime minister.
But speculation has also focused on the Democratic Party's Dragan Djilas, the popular mayor of Belgrade.
The government will face pressure from the EU to undertake a raft of reforms to clinch accession talks - from overhauling the judiciary to culling the public sector, stepping up the fight against crime and corruption and improving economic efficiency.
Pummeled by the economic crisis in the euro zone, the Balkans' main trading partner, a quarter of the Serbian workforce is now unemployed and the economy will struggle to register 0.5 percent growth this year.
The EU also wants more concessions on Kosovo, where Belgrade feeds a de facto ethnic partition four years after the Albanian-majority territory declared independence from Serbia with the backing of the West.
"I'm not sure they realize how soon they will be tested," said a senior Western diplomat in the region. "The scrutiny will be intense."