The battle lines in unionism's civil war are now clearly drawn. No compromise can be countenanced. The protagonists are committed to a fight to the political death.
For the UUP leader, Mr David Trimble, there is no road back from the Belfast Agreement. If he wins, the political landscape in the North is transformed, and several celebrated political careers reduced to the wastelands. A triumphant Trimble really does spell a last hurrah for Ian Paisley, Robert McCartney, Peter Robinson, Willie Ross, Martin Smyth and the rest of the leading No campaigners.
Should Trimble lose, he is politically dead. History would certainly record him kindly as a man prepared to take colossal risks for peace, but also as a leader who had failed to prepare his constituency and, when the electoral test came, to take it with him.
The consequences of defeat for his party are, at this remove, impossible to quantify or assess. At the very least a change of leadership would be inevitable. But what can be imagined of that alternative political landscape would suggest the possibility of a major realignment within unionism itself.
In those circumstances Mr Jeffrey Donaldson, the most obvious of the UUP antis, would certainly find himself competing for the leadership with Mr Robinson and Mr McCartney.
Several Northern politicians from across the spectrum have more than a little difficulty with the notion that the agreement, with all its potential implications for the peoples of these islands, should come down to an internal unionist affair.
Lord Alderdice, for example, vigorously contests the idea that a Yes vote on Friday can leave a question-mark still over its implementation. The people, after all, would have approved it in a free vote.
And Mr David Ervine and Mr Ken Maginnis have both questioned the nature of a unionist No campaign which, as they see it, implies that the votes of the North's nationalist community carry altogether less weight and worth.
Within the No camp, needless to say, there is no thought given to the likely reaction of people in the Republic, who on Friday are expected overwhelmingly to let their heads rule their hearts when voting to abandon the territorial claim to Northern Ireland, and to copperfasten a political settlement which explicitly acknowledges the legitimacy of the North's present position within the UK.
No sign, either, of anxious introspection about the possible consequences in terms of British parliamentary or public opinion; no obvious fretting about the security of the massive British subvention, or the reaction and future attitude of a British prime minister only one year into power, with a commanding majority of 179 and the near-certainty of a second full term before him.
It is this very obduracy, of course, for which these Ulstermen are renowned. And they doubtless exasperate most of the watching world. But, when the House of Commons debated the agreement shortly after Easter, one sensed them warming to their very isolation, cheered in the knowledge that they had been here before, witnessed cross-party alliances in favour of solutions which promised peace and ended in failure.
In any event, while the watching world will dispute that on this occasion `It's right to say No', the principle of consent indisputably bestows upon them the right to do so. Moreover, by design and of necessity, this democratic exercise gives them two bites at the cherry.
And anyway, and self-evidently, it is clear that not just any old majority will suffice. Nationalists themselves long ago established successfully that political institutions in the North must command the allegiance of both communities. And for this agreement to work clearly requires it to be firmly rooted on both sides.
Some time on Saturday afternoon the dust of battle will settle, enabling the parties to assess their position at the end of the first round. All sides expect a victory for the Yes campaign. The question is by how much? Mr Trimble's deputy, Mr John Taylor, yesterday suggested a 65 per cent Yes vote would do the trick.
But the Strangford MP is an old hand at this, and we might remember the "barge pole" he used in the final stages of the negotiations to assess the chances of success. Just days before Good Friday it was registering something around 20 per cent.
In setting his expectation at 65 per cent Mr Taylor would seem to be preparing for the argument that will follow if, in fact, the Yes vote comes in slightly above that but still short of 70 per cent. Mr Trimble meanwhile reverted to his original assessment that less than 70 per cent would be open to debate and could cause him problems.
However, experienced political analysts in the North think Mr Trimble has actually understated the figure required to show conclusively that a majority of unionists are with him.
There is no absolutely definitive figure. There may be an element of republican opposition not registered in the polls, although the evidence suggests Sinn Fein and SDLP supporters heavily voting Yes. Turnout may be higher than usual, with people who don't usually take part in elections wanting to have their say in the single-issue referendum.
But on the basis of last year's general election results, the combined SDLP and Sinn Fein share of the vote at 40.2 per cent, taken with the Alliance Party's 8 per cent, puts the Yes vote within a whisker of a simple majority.
However, it is on the basis of these figures that Mr Peter Robinson says a 26 per cent No vote would suggest a straight 50-50 split within the unionist camp. In other words Mr Trimble needs something over a 74 per cent Yes vote to show that he has an actual majority of unionists with him.
Certainly the potential problems he acknowledged yesterday would seem to come into play he falls short of his own 70 per cent projection.
If the Yes vote is decisive it will embolden Mr Trimble to take on his leading dissidents, possibly preventing them seeking election to the assembly, and demanding loyalty from those who are selected. It may also infuse his party generally with a feeling of confidence.
If it appears that a majority of the unionist people have voted No then the antis will enter the assembly election campaign with the momentum on their side; Mr Trimble's more doubtful allies will see their doubts confirmed; his most fearful (and ambitious) allies may sense the difference between selection and election; and a party machine which has simply failed to fight the referendum campaign on the streets may discover even less appetite for the streets second time around. Mr Trimble, in short, must win big.