This means the final days of the campaign are expected to see a burst of activity to capture the many undecided Protestant voters.
Academics and other commentators differ on what percentages would indicate a clear majority of Protestant voters in favour of the accord. Estimates range from 65 to 75 per cent.
With the Catholic community solidly in favour of the agreement, it is these undecided Protestants who will be targeted by Tony Blair when he flies to Belfast today in a late push to make pro-accord converts.
Some commentators yesterday claimed to detect a slight swing in favour of the "Yes" campaign, though there is no up-to-date opinion-poll evidence to confirm this feeling.
A poll published yesterday was based on fieldwork carried out between 8 to 10 May, before the "No" vote was boosted by the appearance of the IRA Balcombe Street gang at a Sinn Fein conference.
A reminder of violence came yesterday, when an explosive device was sent to a Dublin tourist office. Although no organisation claimed responsibility, it is assumed to be the work of a loyalist group.
The device, apparently mailed from Northern Ireland, contained a small tube of explosive material packed inside a video case with steel balls. The Irish Tourism Minister, Jim McDaid, linked it to the Northern Ireland agreement, but said the Dublin government would not be intimidated.
Mr Blair has spent an extraordinary amount of time recently campaigning for a "Yes" vote, keeping up a daily stream of comments aimed at reassuring the voters. Today's visit is his third to Belfast in recent weeks.
Speaking on a BBC phone-in programme yesterday, he said he believed there was a real chance of defeating terrorism if there was a "Yes" vote. "My view is that if we get this agreement endorsed in the referendum, splinter groups will find it very, very difficult to operate indeed. If these people go back to violence they will get nothing from the US, from the outside world." He added that he wished to give those allied to the main paramilitary groups an opportunity to become democrats.
Meanwhile, a Dublin high- court judge will today decide whether to allow a constitutional challenge to the referendum in the Irish Republic.
Denis Riordan, from Redgate, Limerick, submitted to the High Court that the method the Dublin government proposes using to change Articles 2 and 3 of the 1937 Irish constitution was invalid under the terms of that same constitution.
Mr Justice Peter Kelly granted Mr Riordan leave to bring the challenge in a judicial review to be heard today. He will also rule on his application for an injunction to halt Friday's poll.
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