On present patterns the Good Friday agreement is in some difficulty because large numbers of Unionists remain undecided on how to vote.
To gain real moral and political authority the agreement needs to secure a majority of both Protestant and Catholic voters. While Catholics are clearly overwhelmingly in favour of the accord, the necessary Protestant majority does not exist at this moment.
The Government is now depending on the emergence of a decisive pro-agreement swing before the vote. Mr Blair's visit is targeted mainly at persuading Unionist doubters to support the agreement.
The Prime Minister will reiterate his message of a year ago when, soon after his election, he went to Ulster and declared "I value the Union".
Faced with the latest polling evidence which shows that a large section of the loyalist community still undecided, he will repeat the assurance he gave to the Unionists when he said: "None of us in this hall today, even the youngest, is likely to see Northern Ireland as anything but a part of the United Kingdom."
Private polling is showing that the nationalist community is strongly in favour of a "yes" vote, but the "don't knows" among the Unionists are running as high as 30 per cent, with the rest of the Unionists equally divided between the "yes" and "no" camps. "There is still a large swathe of don't knows that are yet to be persuaded and that may run right up to polling day," said a Westminster source.
Sinn Fein yesterday voiced concerns that Mr Blair might go too far today in his attempts to woo Unionists, warning that too many concessions might upset republican voters. Mr Blair may well calculate, however, that with Catholic and nationalist votes essentially in the bag, his job is to reach Unionist opinion.
He can be expected to address key Unionist concerns, which centre on the security of the union with Britain and the questions of decommissioning, the early release of prisoners and the future of policing.
Unionist indecision is not due to apathy, since broadcasters report huge audiences for programmes on the agreement.
It is believed that the recent visit to Belfast by Mr Blair and former prime minister John Major, together with the launch of the Ulster Unionist Party's Yes campaign, had a significant impact on Protestant voting intentions. On the other hand, the televising of the rapturous reception given to the Balcombe Street gang at the weekend Sinn Fein ard-fheis has obviously produced a negative reaction among Protestants.
One observer said: "It's volatile out there. People are having difficulty finally making up their minds one way or another, but equally they are listening to what is being said."
The No campaign led by the Rev Ian Paisley has meanwhile stepped up its activities, holding almost nightly rallies all over Northern Ireland to drum up opposition to the agreement.
At Prime Minister's Questions yesterday, Mr Blair condemned the "triumphalism" of men who had been involved in IRA killings at Sinn Fein's weekend conference. "The victims of violence have suffered enormously. We do not forget their suffering. I do believe that the peace agreement gives us the best way forward to ensure that there are not more victims in the future," he said. He repeated his demands that all parties to the peace deal should accept the Good Friday agreement in its entirety. He was responding to fears voiced by Ulster Unionist leader David Trimble that Sinn Fein might take the benefits of Assembly places and prisoner releases, without fully signing up to the deal.
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