By Tsvetelia Tsolova
SOFIA (Reuters) - Bulgarians vote on Sunday in a closely-fought election, with the center-right GERB party challenged for power by Socialists who say they will improve ties with Russia even if it means upsetting the country's European Union partners.
Opinion polls put the GERB party of former prime minister Boiko Borisov, 57, only narrowly ahead of the Socialists, who have seen their popularity rise since the candidate they backed, Rumen Radev, won Bulgaria's presidency in November.
Borisov resigned in the wake of Radev's victory, triggering Bulgaria's third parliamentary election in just four years.
While Bulgaria historically has had strong ties with Moscow, Borisov's GERB party is strongly pro-EU and has supported the bloc's sanctions on Russia over its role in the Ukraine crisis.
The Socialists, led by 47-year-old Kornelia Ninova, have vowed to vote against continuing the sanctions, posing another potential headache for the EU as it grapples with Britain’s departure, the rise of right-wing populists and the future shape of the bloc.
Bulgaria takes over the EU's rotating six-month presidency in January 2018.
"The GERB party, to a much greater extent, will maintain Bulgaria's Euro-Atlantic orientation and integration," said Boriana Dimitrova, an analyst with pollster Alpha Research.
"If Bulgaria begins giving up on participation in a number of EU integration policies, underlining its specific interest and privileged relations with Russia, that wouldn't just put it on Europe’s periphery, it would move it into a different orbit."
A decade after joining the EU, Bulgaria remains the bloc's poorest member and corruption is rife.
The Kremlin's most loyal satellite during the Cold War era, Bulgaria remains a popular holiday destination for Russians lured by its Black Sea beaches and cheap prices, and it is also almost entirely dependent on Russian energy supplies.
Many Bulgarians feel a strong cultural affinity for Russia, with which they share the Cyrillic script and Orthodox Christianity.
More than 6.8 million people are eligible to vote in the Balkan country of 7.2 million.
The latest opinion poll put the GERB party on 31.7 percent and the Socialists close behind on 29.1 percent.
If it retains power, the GERB party is expected to maintain a tight rein on public spending – key to Bulgaria's currency peg to the euro – in contrast to the Socialists who have pledged to raise wages and pensions and expand public spending.
Neither party, however, is likely to win enough votes to govern alone.
They will almost certainly have to court the United Patriots, an alliance of three nationalist parties polling third before the election thanks to widespread anger over the flow of migrants from the Middle East, Africa and Asia trying to reach Western Europe via the Balkans.
On Friday, the nationalists blocked Bulgaria's border crossings with Turkey, saying they would stop Turks who hold Bulgarian passports from trying to vote to sway the election.
(Editing by Matt Robinson and Gareth Jones)