In the first 86 years of Australia’s two-party system, only one leader of a major party lost his job without getting the chance to contest a federal election (John Latham in 1931). In the last 13 years, it’s happened three times — to Alexander Downer, Simon Crean and Brendan Nelson. The states tell a similar story: involuntary mid-term leadership changes are concentrated overwhelmingly in the last 30 years, a large number in just the last decade.

Although comparisons are difficult, the same trend seems to be occurring overseas: political parties are becoming less tolerant of their leaders. South Africa’s Thabo Mbeki is the latest casualty. And in Britain, Labour’s Gordon Brown cannot be confident of surviving until the next election.

Late this evening (Australian time), Brown will make the keynote address to Labour’s annual conference, widely described  as “the most important speech of his career.” Since a brief honeymoon after replacing Tony Blair last year, Brown’s trajectory in the opinion polls has been steadily downhill. If an election were held now, the opposition Conservatives would be set for a landslide; one recent poll gave them their highest vote for 20 years.

Removing a sitting prime minister, of course, is a more serious job than despatching an opposition leader — as Peter Costello discovered to his cost. But Labour is already considering its options, and if Brown’s speech tonight falls flat and the polls don’t improve, his days will be numbered.

Brown’s potential challenger, foreign secretary David Miliband, has so far not put a foot wrong. Last year he allowed Brown to take the leadership unchallenged, despite the urging of many Blairites, evidently judging that Brown needed to be given time to fail. This year he has been openly loyal, while clearly sending the message, as the BBC’s Nick Robinson puts it, that “I’m here if you want me”.

What’s interesting about the comparison between Brown and Nelson is that the problems were easily foreseeable — both men’s failings were well known before they gained the leadership. But in each case, the parties got what they wanted, or rather avoided what they most feared: a continuation of Blairism in Britain, and Malcolm Turnbull in Australia.

After some experience of the alternative, the Liberals decided that maybe Turnbull wasn’t so bad after all. Britain’s Labour Party could easily come to the same conclusion about David Miliband.