By Radu Marinas and Marius Zaharia
BUCHAREST (Reuters) - Romania's centrist opposition Democrat party won the country's first election to the European Parliament on Sunday, an exit poll showed, underscoring its hopes of returning to power in next year's parliamentary polls.
The exit poll by independent pollster TNS CSOP, conducted for Realitatea TV television, showed the Democrat Party of President Traian Basescu winning 31.7 percent of ballots.
The vote for 35 deputies to the European assembly is seen as a litmus test of the popularity of leading political parties ahead of the general election, due to be held in late 2008.
"We are the most important political force in Romania," said Democrat Party president Emil Boc.
Prime Minister Calin Tariceanu's ruling Liberal Party trailed the Democrats with 15.2 percent of the ballots. But their score was a touch above expectations for some 13 percent, showing stable backing over the last year.
Observers had expected the Liberals' standing to be eroded by disappointment over Bucharest's reform record which has lost momentum since Romania joined the European Union in January.
"It is a wonderful evening for the Liberal Party because we managed to overcome expectations," the prime minister said.
Bucharest's centrists, who came to power in 2004 on an anti-graft ticket, were praised initially for introducing broad justice and institutional reforms that won Romania EU entry after botched attempts by other post-communist governments.
However, political bickering and a personal conflict between Tariceanu and Basescu have split the ruling alliance of Liberals and Democrats and led to a policy gridlock.
Brussels has threatened to cut aid due to poor preparation in farming and has criticised the justice sector. EU sources say it is becoming clear Romania is standing still or relapsing in its fight on fraud, a key problem in the Balkan society.
The ex-communist opposition Social Democrat Party won 20.8 percent of the vote.
Low voter turnout of less than 30 percent invalidated a referendum on electoral reform, held alongside the European assembly vote.
Nearly 90 percent of voters supported the plan to introduce French-style voting, meant to make politicians more accountable and curb top-level graft. Under Romanian rules, more than half of eligible voters must cast ballots to make a referendum valid.
Basescu will now have to sign into law a rival plan to introduce German-style voting, backed by the government. The president had refused to sign the draft, pending the referendum.