Voters in Northern Ireland must ignore the siren voices of the diehards

In some respects, two different elections to the House of Commons are taking place. There is the general election that Tony Blair and William Hague both hope to win. And then ­ virtually unnoticed in much of the country ­ an almost entirely separate election is taking place. Its importance cannot be overstated.

Elections in Northern Ireland play very much by rules of their own. (Labour and the Liberal Democrats do not exist; the Conservatives are almost invisible.) This Thursday's vote is crucial. It will be the first Westminster election since the signing of the Good Friday peace agreement three years ago. It is a day of make-or-break: the way that the people of Northern Ireland choose to vote this week could either sustain or destroy the peace process itself.

Above all, it is important that voters in Northern Ireland vote for those parties that have fulfilled their obligations under the terms of the peace agreement. Those parties that have opposed the agreement, or that have declared support for it but have failed to deliver on crucial promises, deserve to be cold-shouldered.

Parties that have been least ready to co-operate have sought to portray themselves as having their respective communities' "best interests" at heart. Thus, Unionist diehards who reject the Good Friday peace agreement and everything it stands for, like to see themselves as the defenders of Unionist interests. They are wrong to do so. Those diehards can only make things worse.

Even many of the strongest Protestant opponents of the new Northern Ireland executive have been surprisingly ready, in practice, to co-operate with the nationalists. They have, in other words, been partly socialised by their experience of power-sharing. None the less, a defeat for David Trimble's Ulster Unionists ­ who currently hold nine of the 18 Northern Ireland seats ­ would be a devastating blow, not just for Mr Trimble but for peace itself. Mr Trimble has described himself as "no quitter"; but he will be under extraordinary pressure if his party loses badly on Thursday. Despite what the alleged defenders of Unionist interests seek to suggest, such a defeat would mean that ordinary Protestants would themselves be losers, if the province returns to violence.

On the other side of the denominational fence, it is important that those who are willing to risk everything for peace should not now find themselves pushed aside. Some nationalist politicians have seemed ready to pay lip-service to the peace agreement while still behaving as though this were all just a protracted game of poker. The stakes, however, are life and death.

John Hume, of the mainly Catholic SDLP, deservedly received the Nobel Prize for his courage in seeking peace in Northern Ireland at a time when many still thought that the prospect of an agreement was mere pie in the sky. The willingness of republican terrorists to move across to peaceful politics is praiseworthy; but more than that is needed. Responsibility for the continued failure to achieve the decommissioning of weapons cannot be shrugged off indefinitely. In every respect, the SDLP's moral authority is strong. If the SDLP is electorally wounded, it will be much more difficult for the republicans and Unionists to find the essential common-ground.

Protest votes ­ in effect, a petulant stamping of the foot ­ are, in ordinary circumstances, almost understandable (if not necessarily logical). In Northern Ireland, however, too much is at stake for such protest votes to make sense. If the parties of intolerance are rewarded, the result could be a disastrous return to the violence that has almost been left behind.