They are to visit Northern Ireland to urge voters - particularly wavering Unionists - to back the proposed settlement in polls north and south of the border on Friday.
Mr Blair this morning will go out of his way to reassure anxious Unionists that the temporary release of "callous murderers", including the IRA Balcombe Street gang and loyalist killer Michael Stone to attend political conferences last week, was not part of the agreement.
"These people are out under provisions agreed by a previous government years and years ago," he says on GMTV.
"And the vast bulk of these prisoners would be out within a few years in any event.
"I understand why people feel a sense of revulsion when they see these people, but it is actually misguided to use that as a reason to vote against the agreement," he said.
Originally, all three main party leaders - Mr Blair, Mr Hague and Paddy Ashdown of the Liberal Democrats - were scheduled to make a joint appeal to Ulster voters, but this was scrapped on "logistical" grounds.
Tory party sources also claimed that Mr Ashdown was unwilling to appear with the other two.
The chances of a "Yes" vote were further bolstered yesterday when leaders of the world's top industrialised nations gave their backing to the agreement.
The G8 leaders, meeting outside Birmingham, welcomed it as a basis for prosperity as well as peace.
Ulster was also given a blunt warning of what would happen if there is a "No" vote on Friday.
Ulster Unionist leader David Trimble said that the agreement was "not ideal" and he has reservations about it, but insisted: "The gains Unionists have made are too important to be sacrificed. Those who sought to unite Ireland by violence have failed. While we do not believe that voting `Yes' will of itself guarantee a long and lasting peace, voting 'No' will guarantee a return to violence."
But as the battle for the hearts and minds of the Ulster people entered the final straight Mr Trimble was heckled by "No" campaigners in Co Armagh.
In Portadown he was met with shouts of "No" by Democratic Unionist Party members who trailed him through the town centre.
There were uglier scenes when he moved on to Lurgan. Placard-waving demonstrators yelled "Judas" and "traitor" at Mr Trimble as he tried to meet shoppers accompanied by party colleague Ken Maginnis, MP.Reuse content