The mere possibility of another change of prime minister has elicited a cacophony of demands for an early general election. Some of these calls are, of course, self-interested, though still widely shared. But there are no clear precedents for what is entirely a political, rather than a constitutional, issue.

When a party in government replaces its leader, there is no need for the new prime minister to call an early general election. Macmillan waited 2¾ years, and Callaghan three years until he was forced to hold one by a Commons vote of no confidence. Douglas Home waited a year, and John Major 15 months, but they were near the five-year limit before an election has to be called. Eden called an election almost immediately after taking office, but the parliament was more than 3½ years old. After succeeding the dying Bonar Law in May 1923, Baldwin went to the polls within six months on the issue of tariff reform, only a year into the Parliament, but lost – an unhappy precedent.

There is, however, no example of having two changes of prime minister in a single Parliament (with the wartime exception of Baldwin, Chamberlain and Churchill in the Long Parliament between 1935 and 1945). In opposition, the Tories twice changed leaders between 2001 and 2005 and the Liberal Democrats have done so twice in the current Parliament.

The sole requirement is that a prime minister can command a majority in the House of Commons. Who that leader is, or whether he or she changes, is irrelevant in constitutional terms. That is why the often-heard argument that Gordon Brown’s premiership is illegitimate because he was unelected is baloney. We live in a parliamentary, not a presidential, system. Mr Brown was elected an MP, just as Tony Blair, or any future Labour leader was, in May 2005 by voters in their constituencies. No one was asked who should be prime minister.

In the current, almost hysterical, antiBrown mood, especially in the blogosphere, there have been strident calls for the Queen to intervene to dissolve Parliament (which she would never do). Such antidemocratic ravings show no understanding either of British history or the position of the Crown. Any monarch who even hinted at such nonsense would destroy the institution. It would make the bitter Australian row over the dismissal of Gough Whitlam by the Governor-General in 1975 look minor by comparison. And, then, there was a legislative stalemate.

The real question is political. You can call it either a mandate or an emotional contract (in the words of the marketing men in No 10). That was the case for an election last autumn, and would be even more so if Labour changed its leader this year. Prime ministers who have not won an election lack the personal authority of those who have.

That is why David Cameron’s call yesterday for an immediate election if Mr Brown steps down has such force. It sounds reasonable. A new prime minister would have to give some indication of when an election would be held, even a vague “next year”. That prospect may help Mr Brown, since no Labour MP wants an election with the party so low in the polls and the economy still on the slide. Nothing concentrates MPs’ minds more than the risk of losing their seats.

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