While such a loss would be by no means fatal, it would represent a seepage of moral authority which the Rev Ian Paisley and other critics would attempt to exploit.
The vote, on the shape of proposed new government departments and linkages with the Irish Republic, is expected to pass the report on the new administration, since it will have the support of the pro-Good Friday Agreement parties which form a majority.
But Mr Trimble, who is in waiting to become First Minister, has always had only a slender majority within Unionism itself, and this is now threatened by the attitude of two of his young assembly members.
One of these, Peter Weir, yesterday announced he would vote against his party today while another, Roy Beggs Jr, refused to reveal his intentions.
Assuming the plan for the new departments is accepted, the scene will be set for intense negotiations centred on arms decommissioning. Sinn Fein will press the government to proceed with the formation of a government, but Mr Trimble yesterday repeated that he will not enter an executive without IRA arms decommissioning.
The Sinn Fein position took a battering with the continuing controversy over what the Irish Prime Minister, Bertie Ahern, meant in a weekend interview. While Mr Ahern has beat a hasty retreat from the suggestion that he believed Sinn Fein should not be in an executive without decommissioning, the episode has clearly shaken Sinn Fein's faith in him.
The decommissioning issue featured repeatedly during yesterday's debate in the assembly, with one former loyalist prisoner accusing some Unionists who insisted on decommissioning of hypocrisy. David Ervine, of the Progressive Unionist Party, the political wing of the illegal Ulster Volunteer Force, said he was "not prepared to see the holier than thou attitude prevail".
"They need to remember, when they talk about honour and integrity and decency, how many of them had long and meaningful debates with me when I was a representative ... of the UVF - in meetings all over this country and, indeed, in some of their houses," he said.
In other sharp exchanges Martin McGuinness took exception to the description "Sinn Fein-IRA" as used by Mr Paisley's deputy, Peter Robinson. Following several points of order, Mr Robinson accused Mr McGuinness of being "a self-confessed IRA man".
Mr McGuinness held up an object which he said was part of a hand grenade used in a recent loyalist attack on nationalists. He said he believed it had been imported from South Africa "with the assistance of British military intelligence".
In a less combative contribution, Mr Trimble's nationalist deputy, Seamus Mallon of the SDLP, called on Sinn Fein to adhere to the Good Friday Agreement in letter and in spirit. He also warned Unionists that no decommissioning could be achieved outside the agreement.