David Trimble privately might acknowledge one thing about the Rev Martin Smyth's leadership challenge: it's honest. He must also fear it may prove rather more clever than it first appears.

While others think to subvert Mr Trimble's position by the imposition of additional policy constraints, the South Belfast MP has chosen to answer Michael McGimpsey's "put up or shut up" demand.

In deciding to run, moreover, Mr Smyth has clearly calculated that David Burnside's motion on the RUC - to be debated at tomorrow's second, emergency meeting of the Ulster Unionist Council - may not have the effect some of its sponsors intend.

The motion would make the UUP's re-entry to the executive conditional upon decommissioning and a British commitment to retain the RUC's royal title. And while Mr Burnside insists it is not part of any leadership tilt, some of his key supporters think it would tie Mr Trimble's hands and deny him any further room for political manoeuvre.

Mr Smyth would undoubtedly share that objective. However, he has plainly judged it will not be achieved if passage of the Burnside motion follows on from Mr Trimble's unanimous re-election.

That judgment is not uniformly shared on the unionist right, and Mr Smyth would seem to have presented some of his colleagues with a dilemma. For some, like Jeffrey Donaldson, it might be acutely personal. For all that he was firmly against a leadership challenge, the Lagan Valley MP will have to calculate the potential implications of all this for his own hopes of an eventually seamless succession.

But for those more definitively opposed to the Belfast Agreement, the dilemma may be resolved swiftly. Whatever the disagreement about tactics, the battle has been joined, and they will readily comprehend the likely consequences should Mr Smyth do badly.

The challenger has hardly maximised his chances by declaring just two days before the annual meeting. Moreover (although Mr Trimble's people would be very unwise to play the ageist card), most people until this week will have presumed the former Orange Grand Master is on his way to retirement. It is certainly conceivable that delegates might decide he is palpably not a future leader and consequently reject his challenge quite decisively. Such an outcome could finish off the anti-agreement campaign and breathe new life into the peace process.

Supporters of Mr Donaldson have acknowledged in recent weeks that there is not an anti-Trimble majority within the UUC. At best they reckoned a "stalking horse" challenge would simply confirm what is already known - that Mr Trimble's leadership grip is already tenuous. At worst they feared a situation in which a challenger - stalking horse or otherwise - would fail to match the 42 per cent vote last November against Mr Trimble's decision to enter government with Sinn Fein without a start to IRA decommissioning.

This is the ready-reckoner by which the Trimble camp will invite journalists to assess the success or otherwise of Mr Smyth's challenge. They would be fools not to. However, they would be equally foolish themselves to believe it a reliable or definitive yardstick.

Last November's vote was emphatically not about the leadership, and Mr Trimble would have been the first to reject any interpretation of the 42 per cent as necessarily counting personally against him. Among that 42 per cent must be numbered many Trimble supporters who, while wishing to remain loyal to him, did not want him to make any further concession on the "no guns, no government" policy.

Mr Smyth now threatens to formalise the swirling discontents within unionism around a personal challenge to the leader. He would plainly be delighted to take 42 per cent. However, he can certainly hurt Mr Trimble with less.

In a general, theoretical sense - at least to the Trimbleistas - nothing short of defeat for their man will change anything. Mr Trimble is certainly resolved to defend his political project for as long as he has a majority of one.

However, some of the conventional rules and theories must still apply, even to the Ulster Unionist Party. Mr Trimble is the incumbent. If - after nearly five years in the job, and all the bitterness and division generated by the Good Friday accord - he commands a vote in the high 60s, Mr Trimble will be able to assert that his opponents have made no significant headway since the UUC meeting in April 1998.

But if his challenger (not rated by the Trimble camp as a serious alternative) polls anything much above 35 per cent, Mr Trimble's card will have been marked, and Irish and British hopes for the early restoration of the executive will have been dealt a serious blow.