A tight vote is predicted today as over 2,000 Green Party members vote to decide policy on the upcoming Lisbon Treaty referendum.
The Party's Six TDs and two senators have already declared their intention to campaign in favour of the treaty, expected before the summer.
But over 2,000 members will engage in extensive debate at today's special members' convention at the Hilton Hotel in Dublin to decide the official party position.
The Greens have for many years staunchly opposed greater EU powers, with current leader John Gormley in the vanguard. While the party has been gradually moving towards mainstream politics it remains fiercely democratic and there is no guarantee that the rank-and-file will support the leadership today.
Over half the members are likely to support the treaty but the two-thirds majority needed to endorse the leadership's position will be tight.
This week, two of the party's leading lights, Patricia McKenna and Deirdre de Burca, took opposing positions in The Irish Timesopinion pages, with the former urging members to reject the treaty.
The leadership have proposed three motions - one in favour, one against, and a third motion that asks members to recognise that Ministers "are obliged by virtue of collective Cabinet responsibility to facilitate ratification of this treaty" but acknowledges the right of members to dissent.
Ms de Burca said: "Our members have a very important decision to make and I hope that as many people as possible turn up to hear a very full and balanced debate."
The results of the vote are expected at 5pm.