(SOFIA) - On the eve of a European vote, the Kremlin's closest EU-ally Bulgaria is torn between the bloc that provides it with vital investment or Russia, its deeply influential "big brother" and gas supplier.

The European Union's poorest country knows that a new Cold War born from the Ukraine crisis could have a devastating impact on its fragile economy.

Bulgaria, which joined the EU in 2007, is equally vulnerable to a gas cut by Russia, with which Sofia shares economic and cultural ties, and to toughened sanctions by Brussels that would punish its giant neighbour.

This has left Bulgaria's leaders, already politically fragile, in a difficult bind as they struggle to find the middle road that can satisfy both partners.

For now, Bulgaria's EU partners remain wary that Sofia will be able to break away from Moscow's influence in favour of towing the Brussels line.

German intelligence sources, cited by Der Spiegel news weekly, said Berlin and other member states "worry Moscow will use Sofia as a beachhead for its interests and drive a wedge between EU member states".

At recent summits, Sofia has resisted EU sanctions against Moscow, raising eyebrows across Europe.

The Bulgarian public seems to agree: three recent polls have shown that between 40 and 53 percent of respondents are opposed to such sanctions.

This resistance to turning away from Russia is deeply rooted in history. Bulgaria's national holiday celebrates the country's liberation from Ottoman rule by Russian troops in 1878.

The two nations also share many similarities in language, religion and culture.

Under communism, Sofia was Moscow's most loyal satellite and its economic links with its "big brother" have remained strong under the current government, which is backed by the ex-communist Socialists.

But Gallup political analyst Andrey Raychev, thinks that Bulgarians, like the pro-West activists that sparked the crisis in Ukraine, will ultimately choose the EU over the Kremlin.

"Bulgaria is the wife that is obliged to stick by its husband (the EU), even if she knows she will suffer heavy losses," Raychev said.

- Energy grip -

But the battle to win over Bulgarians to the EU cause will be closely fought.

Bulgaria receives more than 85 percent of its gas from Russian giant Gazprom via Ukraine and Moscow has used this for decades to influence policy.

The country's largest oil refinery is owned by Russia's Lukoil and its sole Soviet-built nuclear power plant still runs on Russian fuel.

Russians are also active in the banking, real estate and tourism sectors.

But Brussels also has big cards to play. European aid funds represent a huge 65 percent of all investment in the country and 62 percent of Bulgaria's total trade is with EU partners.

So far throughout the Ukraine crisis, Sofia has stuck with the bloc, even if reluctantly.

Most resistance by Bulgaria to the EU has been over the South Stream gas pipeline project, which is meant to bring Russian gas to Europe while bypassing Ukraine and is seen as key to Sofia's energy security.

Brussels hinted earlier this month that it might freeze the project, with EU parliament calling for it to be ditched outright.

"South Stream is a project of strategic priority," hit back Economy and Energy Minister Dragomir Stoynev, vowing to start construction on the Bulgarian stretch "this summer" as planned by Gazprom.

For Ognyan Minchev, an analyst from the Sofia-based Institute for International Studies, Bulgaria's energy dependence has been purposefully maintained by the country's politicians and big businessmen, whose ties with Russia remained strong.

Analysts said support for South Stream was clearly "anti-Ukrainian," but ordinary Bulgarians -- who remember the winter of 2009, when a Kiev-Moscow price spat left thousands of households shivering without heating -- also back the project.

Bulgaria's troubled loyalties have marked the otherwise dull European election campaign in this EU newcomer, with the Socialists vowing to serve as "a bridge in the relations between Russia and the EU" and the conservatives playing a more pro-European tune.

Both, however, have firmly backed South Stream.