SOFIA (Reuters) - Former air force commander Rumen Radev, backed by the opposition Socialists, has expanded his lead over ruling party candidate Tsetska Tsacheva and is poised to win Bulgaria's presidential runoff on Sunday, two opinion polls showed.

A win for Radev, 53, is likely to move Bulgaria closer to Russia, putting it at odds with its European Union and NATO allies, and trigger months of political instability after Prime Minister Boiko Borisov said he would step down if his candidate loses.

An opinion survey by Gallup International showed support for Radev had grown to 51 percent, while parliamentary speaker Tsacheva, 58, backed by Borisov's center-right GERB party, is trailing with 40 percent.

Another independent pollster, Alpha Research, put backing for Radev at 49.6 percent and gave Tsacheva 39.1 percent, suggesting about a quarter of the electorate in the Black Sea state are still undecided.

Radev, a novice to politics, is winning over Bulgarians who are frustrated with a political elite they see as corrupt and self-centered.

"People had high hopes for a quick, brief transition, prosperity, high living standards," the mild-mannered Radev said in a televised debate late on Thursday. "But this happened for a small part of the citizens only. The disappointment is that there is no justice."

His lead also reflects disappointment with Borisov's government over its failure to push ahead with meaningful reforms to overhaul the graft-prone judiciary in the Balkan country of 7.2 million.

Radev has pledged to support tough border security measures to prevent an influx of migrants and to draw lessons from neighboring Romania's greater success in tackling corruption.

In Bulgaria, the government and parliament wield most power but the president can veto legislation once, appoints some key officials and is also chief commander of the armed forces.

(Reporting by Angel Krasimirov and Tsvetelia Tsolova; Editing by Gareth Jones)