BUCHAREST (Reuters) - Romania's hard-right nationalist George Simion and centrist Bucharest mayor Nicusor Dan are neck-and-neck ahead of a May 18 presidential run-off that could have far-reaching consequences for the European Union, an opinion poll suggested on Tuesday.

The eurosceptic Simion decisively won the first round of the ballot on May 4, with some 41% of the vote, triggering the resignation of leftist Prime Minister Marcel Ciolacu, the collapse of his pro-Western coalition government, and significant capital outflows.

Simion, 38, opposes military aid to neighbouring Ukraine, is critical of the EU leadership and says he is aligned with U.S. President Donald Trump's Make America Great Again movement. He has suggested he would veto Brussels military aid to Ukraine, while saying Europe should depend on NATO for its own defence.

Dan, 55, running as an independent on an anti-corruption platform, won 21% of the vote in the first round, advocating for continued support for neighbouring Ukraine and alignment with Brussels on key economic and defence issues.

Romania's president has a semi-executive role that includes commanding the armed forces and chairing the security council that decides on military aid. The president can also veto important EU votes that require unanimity.

The poll conducted by AtlasIntel and commissioned by local news website HotNews.ro showed Simion and Dan would each get 48.2% of the vote.

It surveyed 3,995 people from May 9-12 and had a margin of error of 2%.

The survey did not include the diaspora, where nearly 1 million Romanians voted in the first round, more than 60% of them for Simion.

Analysts have said a Simion victory could isolate Romania, erode private investment and destabilise NATO's eastern flank, where Bucharest plays a key role in providing support to Ukraine as it fights a three-year-old Russian invasion.

It would also expand a cohort of eurosceptic leaders in the EU that already includes the Hungarian and Slovak prime ministers.

An opinion survey conducted by polling institute Verifield and commissioned by Dan last week showed Simion was on course to secure 54.8% of the vote in the run-off to Dan's 45.2%.

(Reporting by Luiza Ilie; Editing by Aidan Lewis)