VILNIUS (Reuters) - Lithuanians vote on Sunday in a presidential election expected to hand a new term to incumbent Gitanas Nauseda, a staunch supporter of Ukraine in its two-year war with Russia, following a campaign focusing on security concerns in the Baltic states.

Polls close at 8 p.m. local time (1700 GMT), with results expected after midnight. If no candidate wins more than 50% - which is likely according to opinion surveys - a run-off vote will be held on May 26.

Nauseda, 59, a former senior economist at a top bank, has the support of 29% of voters, a recent Delfi/Spinter Tyrimai poll showed. His strongest challenger is Prime Minister Ingrida Simonyte, 49, the only woman running, with 14% of support.

Across the region, voters are worried the former Soviet republics that make up the Baltics, now members of the NATO military alliance and the European Union, could be the targets of Russian aggression in the future.

Just over half of Lithuanians believe a Russian attack is possible or even very likely, a ELTA/Baltijos Tyrimai poll has found. Lithuanian intelligence said in March Russia was on track to strengthen its military capabilities along its border with at NATO. Moscow has regularly dismissed Western suggestions that it might consider an attack on a NATO member as nonsense.

FOOD STOCKPILES

Most of the candidates running in Sunday's election, including Simonyte and Nauseda, have told the national broadcaster they both keep food stockpiles at home ready in case of a military conflict.

"I am doing all I can to make sure it will not be needed," Nauseda said during a televised debate.

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, to pay for the modernisation of its army and infrastructure to support a brigade of German troops, and their families, that will be deployed in Lithuania and combat-ready from 2027.

While agreeing on Russia policy, the two top candidates differ on issues such as same-sex civil partnerships, which Nauseda opposes, and have a history of acrimonious bickering and refusing to speak to each other.

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

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

In 2019, Nauseda won a presidential election run-off against Simonyte, a former finance minister, with 66% of vote.

Simonyte is also facing a tough test in a general election this October, as government parties trail in the polls.

(Reporting by Andrius Sytas in Vilnius; Editing by Alex Richardson)

By Andrius Sytas