Alan Watkins: An election is the PM's ultimate deterrent

The idea that Mr Miliband or somebody else succeeding Mr Brown should trot off to the Palace seems the height of folly

As Lady Bracknell might have put it to a meeting of the Parliamentary Labour Party: to lose one Prime Minister may be regarded as a misfortune; to lose both looks like carelessness. Throughout the week, assorted kitchen-hands have been keeping the saucepan simmering nicely away: here a pinch of salt, there a twist of pepper, now an adjustment to the gas supply, which was in danger of running perilously low.

The favourite remains Mr David Miliband, as much with the commentators as he is with the bookmakers. He enjoys the support of the foolish virgins – I refer solely to their political opinions – of The Guardian newspaper. Then there is supposed to be a group of "Blairites" intent on revenge. They do not include Mr Miliband among their number, for Mr Miliband is displayed before us as representing the future, whereas Mr Stephen Byers, Mr Alan Milburn and Mr Charles Clarke are depicted as the ghosts of Christmas past. Quite why Mr Clarke should wish to restore the reign of Mr Tony Blair remains mysterious, to me at any rate. He was, I thought, a good Home Secretary in a thankless job – better than his predecessors or those who have succeeded him – and he was sacked as a sop to the Daily Mail on the part of Mr Blair. As for the rest, Mr Alan Johnson does not want the job, or so his friends now tell us.

Mr Jack Straw is always with us. If Mr Gordon Brown were to become "permanently unavailable" (the phrase used in the party rules), the Cabinet and the National Executive Committee would, according to the rules, have to choose an acting Prime Minister. There would then have to be a party election, brought about by a card vote at the party conference. Mr Straw would probably emerge as the acting Prime Minister, though Mrs Margaret Beckett has been touted as a temporary replacement, which may give you some idea of just how silly the silly season has become.

Ah, "emerge"! That was the good old Tory word for a party fix. The last Prime Minister to emerge in this way was Alec Douglas-Home in 1963, though Mr Brown in 2007 ran him close. In 1976 the Parliamentary Labour Party elected a Prime Minister while it was in Government. The episode caused surprisingly little fuss at the time. The chief element of surprise lay in Harold Wilson's resignation of that year.

James Callaghan was seen as the natural, the proper successor. Norman St John-Stevas, constitutionalist and monarchist, made a few ritual noises about the loss of the Royal prerogative to choose a Prime Minister. That was about all. There was certainly no call for a general election.

In 1990, after the fall of Margaret Thatcher, things were a little different. Douglas Hurd, one of the unsuccessful candidates to succeed her, said not only that she should not have been removed in this way but also that, if she had been, there should have been a general election. Even so, Labour was largely content to let the Tories simmer in their own bile and to look forward – wrongly as it turned out – to victory at the polls.

Mr David Cameron was also reluctant to call for an election after the succession of Mr Brown. That was because he thought he would lose. Mr Brown's failure to go to the country in October last year is now put down as one of the great political errors of modern times. For myself, I am not so sure: I still think Mr Brown would have lost, though more narrowly than he would today.

The cry of the moment – at any rate, of the week – is that, if Mr Brown goes, there must be a general election within weeks or, an outside limit, months. On a simple view of law and practice, the cry is wrong.

Mr Miliband, or whoever the new leader turns out to be, can remain Prime Minister till June 2010, or whenever he chooses to go to the people. After all, Neville Chamberlain in 1937 and Winston Churchill in 1940 both became Prime Minister without any need to consult the voters. This precedent is perhaps misleading because of the war. Moreover, Chamberlain was leader of a supposedly "National" government, and Churchill of a coalition. Throughout the last century and from the beginning of this one, we have confined ourselves to one change of Prime Minister for any span between elections.

There is a more fundamental objection to changing Prime Ministers in mid-stream, irrespective of whether it is a first switch or a second. In the Conservative Party, Prime Ministers were changed according to the skulduggeries of high politics. Labour considered itself lucky to have a Prime Minister at all.

For party leaders, the Conservatives introduced party democracy in 1965. Labour had always possessed it. In both cases it was based not on party members but on members of Parliament. And so Labour produced Callaghan and the Conservatives produced John Major. The MPs gave us their majority choices.

It would, I suppose, be perfectly fair to say that Sir John had been "imposed" on the electorate, when Michael Heseltine was clearly the more popular choice. But at least he had the support of his parliamentary colleagues, as Callaghan did before that.

The second phase of party democracy – starting with Labour in 1981, the Conservatives after 1997 – has given additional powers to the party outside Parliament. For example, should Mr Brown become "permanently unavailable", why should the National Executive Committee play any part at all in the appointment of the next Prime Minister, if only on a temporary basis? But so it is.

Apart from this, it is the party conference, not the Labour MPs, who must "trigger" the opening of the contest. And it is the constituency parties and the trade unions, not the parties' MPs, who hold two-thirds of the voting power for the appointment of Prime Minister.

It has so far been forgotten that it is Mr Brown, not anybody else, who holds the power to have a general election. The Victorians, true, used to decide collectively as a Cabinet whether to go to the country. Sir John Major once threatened to have an election when he was having trouble with his party. One of his colleagues said: "We wouldn't have let him get halfway down the Mall." Stanley Baldwin in 1923 defied both his Cabinet and George V and went on to lose the subsequent election.

The idea that Mr Miliband or somebody else should succeed Mr Brown and then trot off to the Palace strikes me as the height of folly. Edward Heath's appeal to the people in February 1974 would seem, by contrast, to be an act of cruel sanity.

The threat of an election cannot be used by his enemies against Mr Brown. On the contrary: the position is reversed. Mr Brown can use the threat against them. I do not think he would bring down the temple. But the thought might still be in his mind, for defensive purposes.