The Ulster Unionist leader found himself in unlikely company in the Commons late on Wednesday. Through the long hours of debate on the Northern Ireland (Sentences) Bill, it was clear anti-agreement unionists were determined to force a vote.
The Bill - paving the way for the accelerated release of some 400 prisoners in the North - effects one of the most contentious aspects of the Belfast Agreement. Being in principle in favour, the Conservatives had signalled their intention not to oppose the Bill at Second Reading - while reserving the right to table amendments and, depending on their fate, to oppose the measure at its final stage next week.
The anti-agreement unionists had no cause for such reticence. Dr Paisley, Mr Robinson, Mr McCartney and their dissident Ulster Unionist allies are opposed to the Bill in principle, and would take the appropriate parliamentary opportunity to register their opposition in the division lobby.
What they could not have anticipated was that they would be joined in the No lobby by Mr Trimble. In one fell swoop the UUP leader brought confusion to his political friends.
There was some suggestion yesterday that Mr Trimble's decision came only as the division approached and his deputy, Mr John Taylor, decided to act as a teller for the Nos. Mr Taylor acted after failing to win a satisfactory answer from the Security Minister, Mr Adam Ingram, that actual decommissioning of weapons would be required before prisoners could benefit from early release. On one interpretation Mr Taylor, by acting as a teller, was firing a warning shot at the government without actually voting against the Bill.
Whatever about that, the effect was another apparent Ulster Unionist split. Because while Mr Trimble marched into the lobby behind Mr Jeffrey Donaldson, Mr William Thompson, and the Rev Martin Smyth, his security spokesman, Mr Ken Maginnis, got smartly offside and did not vote.
Mr Maginnis's abstention is what would have been expected. Certainly there was nothing in Mr Trimble's very able and constructive speech to indicate that he would be voting against the government. His dissident colleagues were clearly surprised. And if there was any element of pleasure at finding themselves on the same side, it was well concealed by a high level of private derision.
Throughout the debate the antis, including Mr Trimble's nominated spokesman, Mr Donaldson, repeatedly charged that Mr Blair had secured the Yes vote in the referendum by way of pledges he could not deliver. The Bill as presented, they maintained, confirmed their claims that the Blair pledges could not be made explicit without requiring the Prime Minister to renege on the agreement.
Mr Trimble, by contrast, just last Friday had welcomed the Bill because it "does contain the four conditions the Prime Minister spelt out in his speech at Balmoral." Yet here he was, five days later, seemingly confirming that - on this issue at least - Dr Paisley, Mr McCartney and company had got it right.
Mr Trimble had, of course, left open an escape route, insisting he, too, would seek amendment to the Bill to make it "watertight". But Mr Peter Robinson charged yesterday: "Voting against a Bill at Second Reading is, in Parliamentary terms, to vote against the principle of the Bill. Mr Trimble's vote cannot therefore be interpreted, as he now attempts to do, as a disagreement about some of the detail of the legislation. A vote against the Second Reading represents fundamental opposition to its core principle and purpose."
Thus the DUP deputy leader was left cheerily to complain that Mr Trimble had "dishonoured" his pledge to work in good faith "to ensure the success of each and every one of the arrangements to be established" under the Belfast Agreement.
And who could deny Mr Robinson his bit of fun? Certainly not Mr Trimble. For the Ulster Unionist position is, at the very least, a mess. The Bill does effect a vital element of the agreement to which Mr Trimble appended his signature on Good Friday. The time for detailed scrutiny and amendment is during next week's Committee Stage. And, even if the UUP takes a different view of the parliamentary niceties, it would seem reasonable to expect that Mr Maginnis - one of only three UUP MPs supporting the agreement - would have been signedup to his leader's chosen course.
As it was, Mr Maginnis - who is directing his party's Assembly election campaign - was left yesterday insisting that Mr Trimble had voted as he did out of annoyance at Dr Mowlam's poor performance at the dispatch box on Wednesday night.
The loyal Mr Maginnis may have increased his claim to an award for services to the art of spin doctoring. But it won't wash.
By any objective assessment Mr Trimble appears once again to have impaled himself on the decommissioning hook. The open question is whether he has done so deliberately.
British government sources yesterday were taking comfort from the notion that Mr Trimble, acting in concert with the Conservatives, may be hoping to win some further concession. But the risk in that strategy was revealed by their doleful conclusion that there may be little more Mr Blair can offer.
There might be scope for some beefing-up of the review mechanisms to attend the release of prisoners and the co-operation of paramilitary organisations with the decommissioning body. But Mr Trimble has said there can be no fudge on this issue. And the fudge will surely be blown sky high if, as seems likely, the government votes down amendments from Dr Paisley and Mr Donaldson seeking to define that "co-operation" in terms of the prior commencement of decommissioning as the price for prisoner releases.
The SDLP leadership has been conspicuous in its efforts to avoid anything which might further inflame already volatile unionist opinion. But Mr Seamus Mallon warned the Commons on Wednesday night that no one could seek to reinstate preconditions which had been canvassed during the negotiation process and rejected.
Mr Trimble has repeatedly warned Sinn Fein it cannot be allowed to "cherry pick" the agreement. Mr Mallon was telling Mr Trimble in no uncertain terms that he likewise will be denied that luxury.