Dublin ? Ireland’s political establishment is waging a high-powered campaign to persuade voters to approve hundreds of pages of international legalese, otherwise known as the Treaty of Nice, that would open the European Union to a vast expansion into Eastern Europe and the Mediterranean.

The yes camp for today’s vote, which includes Ireland’s largest political parties, says the EU’s planned expansion to 400 million people in 25 nations by 2004 is now up to voters in tiny Ireland, population 4 million. Ireland is the only one of the 15 EU nations whose laws didn’t allow simply putting the Treaty of Nice to a parliamentary vote. Because its passage here means amending the constitution, the vote must be put to the people.

This is the second time Irish voters have been asked to vote on the treaty, known simply as Nice. They defeated it in June 2001, a result that no one in official Dublin had expected. Polls just days before had predicted victory. That defeat was “a wake-up call,” Eamon O Cuiv, minister for rural, community and family affairs, said during a debate late last month.

Opponents say the measure would threaten Irish neutrality, promote militarism and reduce the influence of Ireland and other small EU member states. Moreover, they say, its defeat won’t halt EU expansion.

In an opinion poll published Thursday in the Irish Times, 19 percent of voters surveyed said they don’t know how they’ll vote and 10 percent said they don’t plan to vote. Only 24 percent said they have a “good understanding” of the issues on the ballot. Still, 42 percent of those polled said they’d approve Nice and 29 percent said they wouldn’t.

Government and business leaders are determined not to let approval of the treaty elude them again. Yes forces have outspent their opponents by an estimated five-to-one margin, and their ranks include some of the big guns of Irish politics past. Former prime minister Garret FitzGerald, who was leafleting on a Dublin street last week, came out of retirement to promote the issue. O Cuiv, the grandson of Eamon de Valera, the country’s first prime minister, has said that while he voted no the last time, this time he’ll vote yes.

The yes vote has been helped by a steady stream of visits by dignitaries from some of the 10 countries most likely to join the EU in 2004 if Nice is approved here: Latvia, Estonia, Lithuania, Poland, Hungary, Czech Republic, Slovakia, Slovenia, Malta and Cyprus.