Mr Mandelson has offered to address UUP constituency associations to try to boost party leader David Trimble's attempts to win backing for the nationalists' offer.
His proposal has raised hackles in some quarters. Seamus Close, deputy leader of the Alliance, told BBC Radio Ulster that the Northern Ireland people "do not like Big Brother or Big Sister, whoever he or she may be, coming and telling them, or trying to tell them, what they ought to do".
Unionists are split on whether to accept the deal, despite Mr Mandelson's assurances on the decommissioning of terrorist weapons. Some want a "Plan B" in any deal, which would allow the ministerial executive to continue without Sinn Fein if no IRA guns were handed over.
But, in an article for the Independent on Sunday, the senior negotiator of the Progressive Unionist Party, David Ervine, said it was vital for Northern Ireland that Mr Trimble won the row inside his party over the proposed deal.
He argues that Ulster had a "greater need than the removal of guns and that is the need to divest Irish Republicans of the political ammunition which sees Northern Ireland as a `Protestant state for a Protestant people' - and all of the historical baggage which accompanies such a vision of this place".
Ulster Unionist deputy leader John Taylor, who yesterday came down against the proposals, said: "Mr Mandelson is a very good spin doctor and can express things in a way that sounds good. But in reality he is continuing to allow the power of veto, firstly to the IRA and secondly to the SDLP as to the future of the Executive."Reuse content