By Luiza Ilie

BUCHAREST (Reuters) - Centrist Bucharest Mayor Nicusor Dan gained a slight lead over hard-right eurosceptic George Simion, an opinion poll on Friday showed, ahead of Sunday's presidential run-off vote seen as a test of the rise of Donald Trump-style nationalism in the European Union.

Simion, who opposes military aid to neighbouring Ukraine and is critical of the EU leadership, decisively swept the first round of the presidential election rerun on May 4, triggering the collapse of a pro-Western coalition government which led to significant capital outflows.

A Simion win in Sunday's run-off could isolate Romania, erode private investment and destabilise the eastern flank of the North Atlantic Treaty Organization, diplomats and analysts have said.

Dan, who pledged to clamp down on corruption, is staunchly pro-EU and NATO, and has said Romania's support for Ukraine is vital for its own security against a growing Russian threat.

The poll by AtlasIntel, commissioned by local news website HotNews.ro, showed Dan would get 48.7% of votes to Simion's 47.8%. Some 1.8% of those surveyed said they would nullify their vote while another 1.7% were undecided.

The survey of 5,628 people from May 13-15 had a margin of error of 1%, suggesting a very tight race. More importantly, the poll included the sizable diaspora vote, which strongly supported Simion in the first round.

Voting in the diaspora has already begun and nearly 180,000 Romanians have cast their ballot, almost double the turnout by this time in the first round. An AtlasIntel poll earlier this week showed Dan and Simion neck-and-neck.

SWING TOWARDS RUSSIA

Romania had looked poised to swing towards Moscow after far-right newcomer Calin Georgescu topped the first round of voting in the presidential election in November. But the country's top court cancelled the election because of suspicions of Russian meddling, which Moscow has denied.

Simion, who came fourth in the cancelled ballot, rallied behind Georgescu – who has been banned from standing again - and became his replacement. He has said he would appoint Georgescu as prime minister.

On Friday evening, Georgescu told private television station Realitatea that he wanted the job while suggesting an openness towards Russia.

"We have a very detailed plan to take back everything the EU owes us," he said appearing next to Simion. "We will double, triple (EU) fund absorption."

"You must get along well with the great powers. America is first, the main partner in all. But beyond that ... we have China, Russia, Brazil. It is ... unimaginable to have adversarial relations; one must have relationships based on minimal respect."

Georgescu, who has said he favours nationalizations and preferential treatment for Romanian-owned companies, is under criminal investigation on charges of belonging to a fascist group and campaign funding irregularities.

(Reporting by Luiza Ilie; Editing by Richard Chang)