In order to win on the first ballot, Mr Major has to secure not only an overall majority, but also a majority over his nearest challenger equal to 15 per cent of all Tory MPs - a more difficult hurdle than 15 per cent more votes than the nearest challenger, the criterion before 1975. With 327 Tory MPs, this means the Prime Minister must gain 49 more votes than his leading challenger. It was this 15 per cent requirement that defeated Margaret Thatcher in 1990. She was just four votes short of a majority of 56 (equal to 15 per cent of the 372 Conservative MPs at that time). So she "lost" the election even though she polled 204 votes to Michael Heseltine's 152. And her vote was 19 more than Mr Major was to secure a week later when he was elected leader.
Moreover, since new candidates can enter on the second ballot, Tory MPs voting for a first round challenger are in effect supporting not that challenger, but Michael Heseltine, Michael Portillo, Kenneth Clarke, or whoever their favoured candidate might be. The first ballot is thus a contest between the Prime Minister and allcomers. In 1975 there were not, before the first ballot, 130 Tory MPs who positively favoured Margaret Thatcher for the leadership. Similarly in 1990 Michael Heseltine gained 152 votes on the first ballot but only 131 on the second: the 152 were voting not so much for Heseltine as for anyone other than Thatcher. On both occasions, however, the ballot succeeded in its purpose of determining whether the leader retained the confidence of the party.
The electoral system thus favours an insurgent candidate while the sitting leader, has an enormous hurdle to surmount. Even, however, if John Major wins on the first ballot, he can hardly survive a high adverse vote for long. Indeed in 1989, the only occasion when a leader has survived in office under this electoral system, Margaret Thatcher was seriously wounded by the fact that 60 Conservative MPs - more than one-fifth of Conservative back-benchers - declined to support her, just two and a half years after her third election victory and despite her parliamentary majority of 94.
Then, the consequence was a second contest a year later. Today, however, Conservative MPs will hardly wish for a further leadership contest in November 1995 or 1996. Nearly 100 MPs are either members of the government or Whips. If, say, 60 back-benchers - that would be more than a quarter of them - decline to support John Major, how long can he remain as leader in the face of so public an indication of lack of confidence?
The electoral procedure, introduced in 1965 and modified in 1975, was designed to codify the party's attitude to leadership. Conservatives remain, despite Margaret Thatcher, a non-ideological party, a party of government. The vast majority of Conservative MPs are concerned not with divisions between left and right, and still less with the metaphysical niceties of monetary union, but with their chances of electoral survival in the face of an unprecedented deficit of 39% in the opinion polls. To overcome this deficit, a Tory leader needs to command the loyalty of virtually every individual in the parliamentary party.
The electoral machinery is designed to ensure not just a majority choice, as with Labour, but the choice of a leader whose breadth of support precludes the emergence of factions of determined opposition to his or her leadership.
Moreover, previous leadership elections have shown that Tory MPs will pay more attention to disaffected supporters than to the loyalists in their constituency associations. These supporters were crucial, both in 1975 and 1990, for the disintegration of Heath's and Thatcher's regimes began in the constituencies, not so much among activists (Tory associations were five to one for Margaret Thatcher even after the first ballot in 1990) but among those who might be weaned back with a change of leadership. Party influentials - Cabinet ministers and the 1922 Committee's executive - were the last to warn the leader of this disaffection, since their positions often depended on the patronage of the leadership. As Enoch Powell once put it: 'You lose the public, you lose the press, you lose the party in the House, but the men whose heads you can cut off before breakfast you lose last.'
The great priority Conservatives give to party unity dictates the nature of Tory leadership. Fitness to govern rather than ideological rectitude is the key criterion, so a Tory leader enjoys more freedom of manoeuvre but a more precarious tenure than his Labour counterpart. The Tory attitude to leadership was once beautifully expressed by Field Marshal Hakim Amer of Egypt. He was, he declared, perfectly loyal to President Nasser until the day for disloyalty arrived. Disloyalty remains, as it always has been, the secret weapon of the Conservative Party.
Douglas Hurd has said John Major's decision to submit himself for re- election was "a brave decision by a brave man". The term "brave" is part of the vocabulary of political euphemism. The Foreign Secretary, however, has made the Prime Minister appear even braver by announcing his own projected resignation. This makes it almost certain that, even if Major survives the first ballot on 4 July, the disintegration of his regime cannot be long delayed. The key question is not whether that disintegration occurs, but how and, in particular, whether it will make possible the subsequent resurrection of the Conservative Party.
The writer is Reader in Government, Oxford University. His essay on Tory leadership contests is to be found in The Conservative Century, Oxford University Press 1994, edited by Stuart Ball and Anthony Seldon