Mr David Trimble is expected to seek detail from Sinn Féin president Mr Gerry Adams about what the IRA would deliver to ensure a political breakthrough when they meet in London this afternoon.
The British and Irish governments and other talks' participants are placing considerable store on today's meeting between the Sinn Féin and Ulster Unionist leaders - their fifth encounter in recent weeks.
The negotiations reach their most intensive and critical period against a mood of caution and anxiety, with some Ulster Unionist, SDLP, London and Dublin sources expressing concern over whether the IRA will provide a copper-fastened commitment to cease paramilitary activity.
"The nexus required to trigger a deal are autumn elections, the IRA signing up to Paragraph 13 of the joint declaration and ending all activity, and the Ulster Unionists pledging that they will stabilise the institutions of the Good Friday agreement," said one Irish source.
"Anything short of that will leave us in a stop-start situation, and the trouble is that when we stop we may not be able to start again."
Dublin, London, UUP and SDLP sources said that all three elements were important, but the big question-mark at the moment was over whether the IRA would illustrate that its "war is over".
And this was why, they said, today's meeting between Mr Adams and Mr Trimble was of considerable significance.
"We are worried that the IRA can't or won't deliver what everybody knows is required, and that is why David will want to know from Gerry what is the colour of the IRA's money," said a senior UUP source.
Some official and political sources also expressed concern that republicans might fall short in what they deliver in the expectation that with "half a deal" the British Prime Minister, Mr Tony Blair, would still call elections.
Some official and political sources said Mr Blair might indeed call elections in such a situation.
However, a number of UUP sources predicted that without IRA acts of completion the Prime Minister would again postpone elections and put back any renewed attempt at a breakthrough to the spring.
Sinn Féin's chief negotiator Mr Martin McGuinness said on the fringes of the Labour conference in Bournemouth yesterday that there were only "10 days to a fortnight" to break the political deadlock.
"People have been focusing on a singular demand on the IRA, but that focus is both dishonest and unhealthy. The reality is there cannot be a singular focus on the IRA."
The British government must deliver on issues like policing, justice, equality and human rights, while the UUP must ensure the stability of the institutions and accept the need to transfer policing and justice powers from Westminster to Stormont, said Mr McGuinness.
Former SDLP minister Dr Sean Farren, however, said last night: "Complaints that too much attention is being given to this requirement to end paramilitarism have no substance."
Meanwhile, Mr Trimble went into Mr Peter Robinson's East Belfast constituency last night to launch a broadside at the DUP and to also warn that these negotiations could fail.
"It is still possible that the republicans will be unable or unwilling to take leave finally of violence and paramilitarism. Yet because we are clearly in the endgame the DUP are desperately hoping that they can exploit the difficulties and pains of the transition in order to snatch the spoils of our successes," he told the East Belfast UUP association.
"Their problem, however, is that they have never yet been able to accept responsibility, never yet been able to lead. They seek a 'renegotiation' (of the agreement) but have never successfully conducted a negotiation," said Mr Trimble, who accused the DUP of losing its nerve in negotiations.
"They say they want changes but have produced no alternatives, no policies."
The Alliance leader, Mr David Ford, said the first and most fundamental question to be answered before the political process could be restored was that republicans indicate that there would be an end to all forms of paramilitary activity.
