* Centre-right victory tinged by Socialist surge
* GERB party may seek coalition with nationalist alliance
* United Patriots to test GERB campaign promises
By Tsvetelia Tsolova
SOFIA, March 27 (Reuters) - Bulgaria's centre-right GERB party faced the prospect on Monday of lengthy coalition talks after an election victory tinged by the strong showing of pro-Russian Socialists and anti-migrant nationalists.
GERB leader Boiko Borisov, 57, whose resignation late last year triggered the snap election, was due to chair a meeting of his party leadership later in the day.
With 99 percent of votes counted, GERB was seen taking 96 of parliament's 240 seats, leaving it short of a majority and almost certain to seek a deal with the third-placed United Patriots, an alliance of three nationalist parties expected to take 27 seats.
The United Patriots, however, have their own internal rifts over policy towards Russia and the European Union, complicating talks. The alliance will drive a hard bargain, given they could also switch support to the second-placed Socialists, who saw their own share of seats surge to an expected 79.
The Socialists, capitalising on their victory in a presidential election in November that triggered Borisov's downfall, have vowed to improve ties with Russia even at the expense of EU unity.
"It is possible to make a government with GERB; it is possible not to," Valeri Simeonov, co-leader of the United Patriots, told Bulgarian Nova TV on Monday.
"It all depends on whether we can agree on policies. Last time around it took us about a month."
The United Patriots have capitalised on a growing mood of nationalism in Bulgaria since hundreds of thousands of migrants from the Middle East, Asia and Africa began crossing the Balkan peninsula en route to Western Europe two years ago.
The alliance is staunchly opposed to immigration and has called for legislation to address crimes committed by Bulgaria's Roma minority, to increase pensions and keep down electricity prices. GERB has pledged to maintain the tight fiscal policies underpinning the lev currency peg to the euro.
A role in government for the United Patriots may also further worsen Bulgaria’s strained relations with neighbouring Turkey.
Borisov may also seek to bring the populist Will party, which is estimated to have won 12 seats, into a three-party coalition or try to lead a minority government.
Whatever the outcome, analysts were sceptical the election would produce a stable government – the country's seventh since 2013 – capable of tackling widespread poverty and corruption as Bulgaria takes on the rotating presidency of the EU in January 2018.
"The election did not give a clear mandate for stable, long-term governance," said Daniel Smilov, a political analyst with the Sofia-based Centre for Liberal Strategies.
"The next government will have a horizon until July next year, when the country will hand over the EU rotating presidency. In terms of reforms, we are likely to see more of the same." (Editing by Matt Robinson and Janet Lawrence)