At least five of his ten MPs have emerged as opponents of the deal, with others in the party hierarchy also hostile, including his predecessor as leader, Lord Molyneaux.
Mr Trimble appeared initially to have won a solid endorsement for the new deal which he played a crucial role in putting together. But with an array of opponents now emerging, he will clearly have a fight on his hands as he seeks party endorsement.
On Saturday, his party's executive committee voted by 55-23 in support of the agreement, recommending it to the much larger meeting of the party council which will take place at the end of the week.
The vital vote on the agreement is scheduled for Saturday next, when around 800 members of the ruling Ulster Unionist Council are to give their verdictat a special meeting in Belfast. The opinions of the MPs are not vital on such occasions but they carry great weight.
The deal - and indeed any participation in the peace process - has been opposed for some months by MPs William Ross, William Thompson, Roy Beggs and Clifford Forsythe. It was also belatedly opposed by Jeffrey Donaldson, who left the talks building on Friday shortly before the deal was concluded.
Mr Donaldson was one of party's chief negotiators at Stormont and had been regarded as one of Mr Trimble's closest aides before swerving sharply in recent weeks. Mr Trimble said on Saturday he was encouraged by an assurance from Mr Donaldson that he would not split the party.
It is now known that a number of the party's negotiating team in the talks also came out against it before Friday's closing plenary session. Most of these are regarded as being from the party's younger and more militant faction.
The sixth MP who is said to be against the deal is the Rev Martin Smyth - a former grand master of the Orange Order, who was in Australia when it was concluded. Although he has yet to confirm it, others in the anti- deal camp regard him as being on their side.
In addition, Lord Molyneaux - who led the party for almost two decades until Mr Trimble took over in 1995 - is also opposed. This means that Mr Trimble can only count on the support of three parliamentary colleagues - John Taylor, Ken Maginnis and Cecil Walker.
While Members of Parliament are not central in the complicated structure of the Ulster Unionist Party, the array of opposition represents a formidable battery of big guns. An important meeting of the Orange Order is to be held in Belfast on Wednesday, and it will come as a blow to Mr Trimble if this comes out against the deal.
Mr Trimble said: "Of course some people have doubts and reservations. Whoever heard of a party where there weren't differences of view? We are not some Stalinist organisation. We are the most democratic political body in these islands. I think people who believe this issue will divide the party are being unduly pessimistic."
On the republican side, the Sinn Fein president, Gerry Adams, yesterday urged all activists to examine the document in great detail, to consider whether "the struggle" had been advanced, and to consider how it could be advanced further.
Sinn Fein is also likely to contain many who will have considerable problems with the document, containing as it does many elements traditionally opposed by republicans. The partyholds its ard-fheis (annual conference) in Dublin next weekend, when the views of the grassroots will become clearer.
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