The two sides outlined their positions in letters sent to party members, writes Dan Keenan, Northern News Editor.
Mr David Trimble has appealed to members of the Ulster Unionist Council to support his resolution calling on the three rebel MPs to resume the party whip and accept the UUC's decisions.
His critics have also written to council members calling for their votes to have disciplinary action measures against them withdrawn.
Both letters have been seen by The Irish Times, and they make clear the different ground on which the UUP battle is being fought.
Mr Trimble's letter, dated September 3rd, was received this week by each of the 900 council members. In it he makes a direct appeal for continued backing for his policy of cautious support for the Joint Declaration - a policy endorsed by the council at its last meeting in June. It was the last UUC vote that prompted Mr Jeffrey Donaldson, the Rev Martin Smyth and Mr David Burnside to resign the whip at Westminster, effectively halving the UUP representation in the Commons.
Mr Trimble goes to pains to outline his position on issues which are at the core of unionist concerns about the Joint Declaration, insisting that the actions of the rebel three are in defiance of the UUC as a whole rather than him as leader in particular.
He paints the actions of the three MPs as being in defiance of the decisions of the council.
Referring to the vote last June to endorse a strategy of "withholding endorsement from the Declaration as a whole", Mr Trimble wrote: "Regrettably, just a week after the Council's decision, three members of the Parliamentary Party including two Officers of the Council, repudiated the Party Whip. They announced their intention of aligning with other unionist parties including combining to fight elections." He accused the three MPs of saying to council members, "Do what we say or we will split the party". Claiming that all unionists had "to play by the same rules" he urged council members to "accept the authority of the Ulster Unionist Executive and the Ulster Unionist Council".
Apparently anxious to deflect claims that disciplinary measures initiated by him were damaging and divisive, Mr Trimble portrays himself as sensitive to the party's concerns and his critics as obstructive.
"I have made considerable efforts over the years to keep the party together," he writes. "I have openly discussed policy with others. I have included prominent critics in key roles I have taken the party into my confidence in a way that no leader before me has done. I included every MP and all Assembly candidates in the group that was planning a response to the Joint Declaration until that process was disrupted." He also blamed the stance of his Westminster critics for taking the spotlight off what he called "republicanism's intransigence".
On that basis, he calls for further support. "It is vital that the ability of the leadership to represent the party is sustained," he writes.
Outlining his amendment to today's resolution, Mr Trimble says his approach offers "a way of drawing back".
"If this path is taken then any disciplinary action would fall by the wayside and we can return the focus to achieving our objectives." In their letter, the three MPs insist that disciplinary action against half of its Commons members is damaging, divisive and ought to be withdrawn.
Mr Donaldson, the Rev Smith and Mr Burnside highlight the fall in the UUP's electoral ratings and point in Mr Trimble's direction. They claim the support of those "deeply disillusioned with the current state of the political process and strongly opposed to the Joint Declaration".
They further claim that 70 per cent of unionist voters agree with them.
Holding up the "best interests of the people whom we have been elected to represent in Parliament" they say that the Joint Declaration is "an affront to democracy and the rule of law".
Concentrating on key unionist concerns about the popular standing of the UUP in the eyes of voters, the MPs' letter returns time and again to the electoral price the party is paying for its open civil war.
"Will the political issues confronting us at this time be resolved by throwing three MPs out of the party?" they ask.
They predict that such a disciplinary measure "holds the prospect of a disastrous and irreversible split in Ulster Unionism which will leave the party as a minority rump at Westminster and no longer the major party in any future Assembly or local administration in Northern Ireland".
"Let us state unequivocally that this is not an outcome that we wish to see happening to our party." Their letter concludes by confirming that talks have been taking place throughout the summer on a possible resolution to the party's divisions. They pledge that talks will continue but only if disciplinary threats are removed.
They ask council members for support today, "in the hope that a positive decision will clear the way for much needed progress on the issues that really matter to our people".