SOFIA (Reuters) - Bulgaria's nationalist Attack party will work to topple the government if Sofia backs a new round of Western sanctions against Russia over its annexation of Crimea, the party's leader told Reuters in an interview on Tuesday.

Leader of Bulgarian nationalist Attack party Volen Siderov (R) arrives at Sofia's investigative service building March 12, 2014. REUTERS/Stoyan Nenov

Attack holds the balance of power in the Bulgarian parliament, and its threat is an extreme example of the domestic pressures European governments face in taking a position on the worst crisis in East-West relations since the Cold War.

If the party pulls the plug on Prime Minister Plamen Oresharski's minority coalition government, it could tip the European Union's poorest member state back into political instability. Months of street protests felled one government last year and almost toppled its successor, at a time when the country was struggling to revive economic growth.

Bulgarian leaders' position in relation to the Ukraine crisis is especially precarious. On the one hand, the country has pushed for closer ties with the EU bloc, which it joined seven years ago. On the other hand, it is almost entirely dependent on Russian energy supplies, and many Bulgarians feel a deep affinity for their giant neighbour across the Black Sea.

Attack, which is a noisy critic of the United States and Bulgaria's NATO membership, is the only Bulgarian party to openly champion Russia's intervention in the Ukraine.

"If the current Bulgarian government supports them (tougher sanctions), I will take a firm opposition line towards it and I will work for its toppling," party chairman Volen Siderov said in an interview in his office in the Bulgarian capital.

"Then we will have to ask ourselves the question what government and what majority will come after it, whether it will

be the same, or whether it can be changed," he said.

"It is a problem for the voters, who have to wake up and finally understand that the government in Bulgaria should have an independent policy and put an end to colonialism that we have been forced into."

As if to underscore Bulgarian ties to Russia, Siderov has antique guns and swords fixed on his office wall - some of them relics of Russia's wars against Turkey in the 19th century that helped Bulgaria throw off 500 years of Ottoman rule.

TAKING A STAND

As well as the threat of Attack withdrawing its support, the ruling Socialist party also risks angering voters in the European elections in May. The elections are seen as a key test for the survival of a government that has already faced down three confidence votes since taking power last year.

A Gallup poll last week underlined the country's divided loyalties: 30 percent of Bulgarians supported Ukraine joining the EU, while 27 percent supported Ukraine forging closer links with Russia instead.

The Socialist party is a direct descendant of the communists who governed Bulgaria before the fall of the Berlin Wall, when the country was seen as Moscow's most pliable ally. Many of its core supporters are pro-Russia.

Its conflicted position has led the government to send mixed signals about its intentions. Bulgaria allowed the first round of sanctions against Russia and twice took part in navy drills with a U.S. warship in the Black Sea. The prime minister said the country would not support further sanctions, but then also suggested Bulgaria would not veto them if imposed.

In any case, a new round of punitive measures might be fiercely resisted by many of the EU's other ex-communist nations, who are fearful of Russia's aggression but have the most to lose by imposing sanctions.

"Bulgaria needs to impose a veto," said Siderov, a white-haired former journalist, who was charged with hooliganism earlier this month after an alleged row with a female French diplomat on a plane. "It has the right to do that and then the European Union will not be able to impose sanctions," he added.

"And silently, and not officially, the German and French representatives will say "well done for saving us", because their turnover with Russia is in tens of billions of euros."