Irish voters headed to the polls Thursday in a historic referendum that will determine the fate of a treaty designed to overhaul the European Union.

The  Treaty of Lisbon would create a long-term president of the Council of EU leaders, a stronger foreign policy chief, a more democratic voting system and greater powers for European parliaments.

Ireland is the only one of 27 EU members holding a vote on the issue. While the other 26 countries have left the issue to their parliaments, Irish law requires that a popular vote be held on any proposed constitutional changes.

The referendum gives Irish voters significant power, even though the Irish population only represents one per cent of the EU's 490 million people. If voters say "No," the treaty falls apart because all 27 EU members must ratify the treaty for it to be approved.

So far, 15 countries have accepted the treaty, while others are still debating the issue in their parliaments.

In Ireland, opinion polls show the "Yes" and "No" sides locked in a tight race.

"It's going to be close, there's no doubt," said Cyprian Brady, a politician with Ireland's Republican Party who has been going door-to-door in Dublin, urging voters to accept the treaty.

"I feel myself, personally, that the "Yes" will swing it, but it's definitely going to be close."

Some fear sovereignty loss

The Republican Party, which is the leading party in the ruling government coalition, supports the treaty, as do other main political parties, but many in the general population aren't convinced, says Raj Chari, a Canadian who directs the Centre for European Studies at Trinity College in Dublin.

Opponents fear the treaty would rob Ireland of its sovereignty and allow the European Union to overrule Ireland's own government on sensitive issues such as the country's strict abortion laws or its long-held position of military neutrality.

"Walking around Dublin, you see lots of signs that say, 'Don't let them bully you into this,'" Chari said.

The treaty, a complicated 300-page legal document that has left some Irish voters confused, replaces a more ambitious draft constitution that was rejected by French and Dutch voters in 2005.

Caroline Simons, with the anti-treaty group called Libertas, hopes Irish people will turn their backs at the treaty.

"Irish people are very feisty," she said. "They don't like to be bullied. There's a huge amount of bullying going on and I think, at the end of the day, people will see that."