So will history repeat itself? For Jim Callaghan read John Major. For gas pipeline (the one a tottering Labour administration offered Northern Ireland MPs in a vain effort to keep afloat in 1979) read cows (the ones Ulster farmers want exempted from the EU beef ban in 1996). Once again it is the Northern Ireland MPs, and above all the nine-strong Ulster Unionists, who may prove to hold the fate of the Government in their hands.

In March 1979 during the confidence vote which brought the Callaghan government down, James Molyneaux led all but two of the Ulster Unionist MPs into the division lobbies against the government. Seventeen years of Tory rule later, it isn't much of an exaggeration to say that David Trimble, the UUP's leader, is beginning to look like the most powerful MP at Westminster. The Government's majority is one and falling. So Trimble has the potential to determine the general election's timing, and therefore, perhaps, its outcome.

There is scarcely an MP in any party who doesn't believe that John Major's best chance of winning is to leave the election until the spring. It follows, therefore, that if he were forced into going to the country before Christmas it would correspondingly reduce further his chances of victory.

One, though by no means the only, reason why Labour chose the issue - super-sensitive in Northern Ireland - of BSE for its debate next week was to test whether Trimble wanted to do the forcing. If Trimble wasn't prepared to threaten the Government on behalf of his especially aggrieved farmers in Northern Ireland, then when would he be?

It isn't difficult to list the reasons for thinking Trimble might decide to send the Government on its way. The first, simply, is the perception that Labour will win. There is a quite a lot of Unionist folklore that Margaret Thatcher was irritated with the Ulstermen for supporting the Callaghan administration as long as they did and that it would have been better for subsequent relations if they had pulled the plug earlier. Why prop up a Tory government if a Labour one is what you are going to have to deal with in a few months' time?

The second is the careful shift which Tony Blair has made in his party's policy from prescriptive nationalism to neutrality on Northern Ireland's future, making the prospect of a Labour government much less intimidating for Unionists. (Indeed, some nationalists even think a strong Tory government would be the likeliest to produce a solution acceptable to them - rather in the way that it was Nixon who recognised Red China.)

The third is that, in protest at the hated (by the Unionists) London- Dublin framework agreement on a cross-border political settlement last year, Trimble publicly abandoned the doctrine laid down by his predecessor, namely that the UUP would not normally bring down a government elected by a majority in Great Britain. Trimble's warnings that he will take every vote on its merits has been, on the face of it, quite menacing for Major.

But how real is the threat? Next week's vote, for a start, looks unlikely to spell the downfall of the Government. Michael Forsyth, the Scottish Secretary, has been fighting a powerful rearguard action against a Northern Ireland exemption from the EU beef ban (which most of Europe is now ready to offer) unless it is extended to Scottish farmers. Neither Forsyth nor his farmers will like it but it looks increasingly as if Britain will grab the offer. But anyway a closer examination of the Major-Trimble relationship suggests that the Unionist leader is ready to see him through the winter.

Major's care in dealing with Trimble should not be underestimated. After meeting the Unionist leader in Bournemouth, he promised Trimble his cherished parliamentary Northern Ireland Grand Committee. The importance of this is baffling to most people in Great Britain; but matching as it does the revamped Scottish Grand Committee, it is a powerful integrationist symbol for Unionists and in the eyes of some a congenial alternative to a power-sharing assembly.

Trimble complained crossly yesterday in his regular weekly meeting with the chief whip, Alistair Goodlad - a fixture which itself testifies to the open access to the Tory hierarchy enjoyed by Trimble - that Sir Patrick Mayhew, the Northern Ireland Secretary, seemed to be stalling on the details of the committee. But this merely suggests that Sir Patrick was not keen on the idea and that it was stitched up between Major, Tony Newton, the leader of the Commons, and one of Trimble's closest friends in the Cabinet, Lord Cranborne. It's worth remembering that for all the switch of policy (and Northern Ireland spokesman) by Blair he has no powerful Unionist- friendly equivalent to Cranborne in the Shadow Cabinet. What's more, those who have recently observed the body language between David Trimble and his fellow Ulsterman, Brian Mawhinney, have been struck by their warmth

Conversely, the honeymoon between the Unionists and Marjorie Mowlam, Blair's energetic Northern Ireland spokeswoman, has also cooled visibly. Trimble has complained about Mowlam's observation that the "status quo" in Northern Ireland is not a long-term option, and there have been murmurings in his party that she still listens to some of those who advised her overtly pro-nationalist predecessor, Kevin MacNamara.

Finally, it's not clear that the UUP is ready for a quick election. They did not do well in the recent elections to the Northern Ireland forum and, with Ian Paisley's DUP - along with Bob McCartney, the Independent Unionist - bitterly criticising Trimble for the concessions he made to allow cross-party talks to continue in Belfast, it may not be the most auspicious time for the Unionists to go to the polls.

For all these reasons, forget Trimble as the catalyst for an early election. It's just possible that if Major and Mayhew secured a renewal of the IRA ceasefire and brought Sinn Fein into talks, then the tough conditions Trimble would set for their participation could bring him into fresh conflict with Downing Street. He could yet lose his patience in, say, February.

But Trimble is determined, canny and hard-line - more so perhaps than his predecessor - in opposing what he recently scathingly called the "Anglo- Irishery" which produced last year's framework document. And a weak minority government remains a good deal more appealing to the Unionist leader than a strong one of either party. If Trimble is anyone's secret weapon in the coming months, he's Major's.