The anti-agreement Ulster Unionist MP, Mr Willie Ross, has warned the two governments as well as nationalist politicians and his own party leader that they would be foolish to write off the No camp.
It may have lost the weekend Ulster Unionist Council vote but he believes victory is inevitable. "The No camp is far too big and powerful to go away. Just give us time. Trimble will run into trouble when the guns aren't delivered. Fifty-three per cent of the vote isn't enough to carry through a major policy change."
Mr Ross claims his leader secured crucial votes by "misleading" delegates into believing actual decommissioning would take place. "When that doesn't happen, those people will reject his deal. Trimble won a pyrrhic victory on Saturday and we all know pyrrhic victories lead to losing the war."
Mr Ross believes the UUP will fare badly in the forthcoming South Antrim Westminster by-election and future council by-elections. A fall in electoral support will cause more party members to turn against Mr Trimble's policies, he predicts.
The No camp is planning another UUC meeting, probably in September or October. It takes only 60 of its 860 members to do so. Mr Peter King, chairman of the Unionist Graduates' Association, says it will be crucial - "It will be the final battle."
After Saturday's vote to re-enter the Executive, he would find it hard to vote for the UUP, let alone retain his membership.
"I just don't want to be associated with these people, but I think Trimble made certain promises and it is up to us to keep the pressure on him.
"Many delegates were swayed by his `nudge, nudge, wink, wink, decommissioning will happen' tactics. He has left himself open for attack when product isn't delivered. There were only 56 votes in it on Saturday. A change of 29 would swing it for us. You don't give up when it is that close."
A former Trimble ally and UUC anti-agreement delegate, Mr John Hunter, isn't so sure the No camp can win from within the party. He points to last week's Banbridge council by-election result - where the UUP vote fell and the DUP vote rose dramatically - as evidence of future electoral defeat.
"The No camp has to accept that the electorate no longer trusts the UUP and believes it has broken its manifesto pledges. Ordinary unionists think the UUP has put expediency before principles.
"Fifty-three per cent of UUC delegates on Saturday accepted the word of an organisation which has murdered party members and friends. That is deeply disturbing and I don't know if the party can be saved."
However, anti-agreement Assembly member Mr Peter Weir believes Saturday's decision can be reversed. "The next few months will be crucial. Many delegates supported David Trimble believing he will secure actual decommissioning. Their feelings may change when that doesn't happen.
"They also believe the Union Jack will be flown from government buildings. I suspect the two Sinn Fein ministers won't allow this in their departments and the SDLP will follow suit. That will annoy many unionists.
"This deal was sold on the basis of things that aren't likely to happen. Unionist disillusionment seems inevitable."
Anti-agreement sources say a key factor will be if the Executive is seen to work for ordinary unionists. "Last time, many ordinary unionists felt it didn't do much for them. If it can deliver a few goodies - whether symbolic or in terms of jobs and investment - that might change," admits one activist. "Everything is still to play for."