VILNIUS (Reuters) - Lithuania holds presidential elections on Sunday, with incumbent Gitanas Nauseda expected to win after a campaign dominated by security concerns in the post-Soviet state.

The Baltic nation of 2.8 million people has been a staunch ally of Ukraine since Russia's 2022 invasion. Like other countries in the region, the NATO and EU member worries it could be Moscow's next target.

Nauseda, 60, a former senior economist with Swedish banking group SEB who is not affiliated with any party, won the first round of the election on May 12 with 44% of the votes, short of the 50% he needed for an outright victory.

He is running against Prime Minister Ingrida Simonyte, 49, from the ruling centre-right Homeland Union party that has been trailing in opinion polls. She was the only woman out of eight candidates in the first round and came second with 20%.

Just over half of Lithuanians believe a Russian attack is possible or even very likely, according to a ELTA/Baltijos Tyrimai poll conducted between February and March. Russia has regularly dismissed the idea that it might attack a NATO member.

Nauseda told a debate on Tuesday he sees Russia as an enemy. "Our enemies - who even call themselves our enemies, who are enemies of us and all the democratic world - are attempting to destablilize our politics, and we must do all to resist."

Both Nauseda and Simonyte support increasing defence spending to at least 3% of Lithuania's gross domestic product, from the 2.75% planned for this year.

But Nauseda, who is a social conservative, has clashed with Simonyte on other issues, including whether to give a legal recognition to same-sex civil partnerships, which Nauseda opposes.

He has said it would make such unions too similar to marriage, which Lithuania's constitution only allows for a man and a woman.

Simonyte, a former finance minister and a fiscal hawk, said on Thursday that if she won, "the direction for the country - pro-European, pro-Western - would not change".

"But I would like quicker progress, more openness and understanding, larger tolerance to people who are different from us", she added.

Lithuania's president has a semi-executive role, which includes heading the armed forces, chairing the supreme defence and national security policy body and representing the country at European Union and NATO summits.

The president sets foreign and security policy in tandem with the government, can veto laws and has a say in the appointment of key officials such as judges, the chief prosecutor, the chief of defence and the head of the central bank.

It will be the second time the two have competed in a presidential run-off. In 2019, Nauseda beat Simonyte with 66% of vote.

(Reporting by Andrius Sytas in Vilnius; Editing by Andrew Heavens)

By Andrius Sytas