A tight race is predicted in Bulgaria's parliamentary election on Sunday as the Socialists, seen as closer to Russia, aim to prevent another comeback by two-times centre-right premier Boyko Borisov.

Opinion polls in the European Union's poorest country, where the average monthly salary is only 500 euros ($540) and corruption is rife, also indicate a strong showing by the nationalist United Patriots, who are tipped to come third.

Forming a government after Bulgaria's third election in four years will be tough, and the resulting coalition may be short-lived, experts say.

Left-wing live-wire

Kornelia Ninova, 48, has injected fresh vigour into the Bulgarian Socialist Party (BSP), successor to the Communist Party, since becoming its first female leader last year.

Moscow and Sofia have deep cultural and economic ties and Ninova says she will veto a renewal of EU sanctions on Russia.

A recent cartoon depicted her on a Trojan Horse with Russian President Vladimir Putin inside.

Bulgaria's would-be first female prime minister, a former lawyer and business executive, has also criticised the EU, which Bulgaria joined a decade ago.

"We are the party that ushered Bulgaria into the European Union and NATO ... But we do not want to be a second-class member," Ninova told AFP news agency in a recent interview.

Ninova enjoyed a major success in November when Rumen Radev, a former airforce commander backed by the BSP, was unexpectedly elected president.

Bounce-back Borisov

Burly former firefighter and bodyguard Borisov, 57, head of the centre-right GERB party, has been the towering figure in Bulgarian politics in recent years.

He has been premier twice but resigned both times, once in 2013 after nationwide protests against poverty and most recently in November after the presidential election.

Like most Bulgarians he also has a soft spot for Russia, giving Putin a puppy in 2010 and calling in the campaign for "pragmatic" ties, but he toes the EU line on sanctions.

And again like many compatriots, he is an enthusiastic European.

Borisov says he wants a third mandate "to guarantee stability". But although surveys suggest GERB may come first, analysts say voters may have had enough of him.

Turkish connection

Mustafa Karadaya, 46, heads the centrist Movement for Rights and Freedoms (MDL) representing mostly Bulgaria's 700,000-strong Turkish minority, a legacy of centuries of Ottoman rule.

The party, expected to garner between 8 and 11 percent of the vote, has however become critical of Turkey and its President Recep Tayyip Erdogan.

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Karadaya says Erdogan has "abandoned" the values of Ataturk, modern Turkey's founder.

Last year a splinter more pro-Ankara group split off and formed a new party, Dost. Turkey has openly supported it, creating a rift with its neighbour.

Kingmaker 'patriots'

A new nationalist grouping, the United Patriots, has jumped on this spat with Turkey to boost its support, while railing against migrants, Muslims, Roma and gay people.

Krasimir Karakachanov, 51, one of its three leaders, came third in the presidential election, and the United Patriots could repeat the feat on Sunday. It might then act as kingmaker in coalition talks.

Karakachanov doesn't question Bulgaria's place in the EU and NATO. But he has called for sanctions on Moscow to be lifted.

'Make Bulgaria great again'

Another player in coalition discussions might be Veselin Mareshki, 49, a colourful populist who likes being called the Bulgarian Donald Trump, and his party Volya.

Like the US president, Mareshki is a businessman and political novice promising to drain the swamp of Bulgarian politics. He came fourth in the presidential race.

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To demonstrate his prowess, his chain of petrol stations are currently selling fuel for 15 percent less than its competitors to show that the sector is a "cartel".

Unstable government

In the ex-communist nation's third election in four years, many voters are turning away from the main parties towards groups on the fringes, or are not bothering to vote.

"I will back neither Borisov nor the opposition Socialists. I do not believe them any more," teacher Tsvetomira Tosheva, 47, told AFP news agency in Sofia.

The resulting government may not last long.

"It seems that some configuration of political parties who support the oligarchic government model will win the elections," political analyst Evgeni Daynov told AFP.

"But because society has already realised how dangerous corruption is, this will inevitably lead to a highly unstable government."