If things go the way the Cameron and Obama households are hoping, then, in around 2009, a US-UK summit will take place between the first black president and the first prime minister known to have smoked pot. Of course, this may not happen. It's possible that the Washington end of the negotiations would be handled by the nation's first woman leader (Hillary Clinton), first Mormon (Mitt Romney) or first double divorcee (Rudy Giuliani).
The moral of these fantasies is that definitions of electability seem to be changing. At the start of any campaign, the team sit down and calculate the candidate's "negatives" (there's a fine West Wing episode in which Josh writes down the pros and cons on Santos). But the electoral cycles beginning in Britain and America tantalisingly feature serious contenders who represent races, religions, domestic arrangements or student ingestion that would until recently have been considered insurmountable obstacles to office.
This is an overdue development. Many politicians and political commentators hold the reassuring view that British and American electorates have a record of sensible decisions. Leaving aside personal ideology and looking at the long sweep of history, some on the left would accept that Labour was not fit to govern in the 80s, while rightwingers might agree that Bob Dole and William Hague were not plausible national leaders when they sought the job.
The weakness in this theory of the knowing voter, though, is that electorates have only had the chance to pick from an alarmingly restricted list of possibilities: almost always featuring white male middle-aged Protestants who accepted the bankers' and the soldiers' definitions of the national interest.
The latter part of the equation won't change. Even if the two countries are governed later in this decade by people who have experienced menstruation, racism or marijuana, or have taken part in more weddings or fewer baptisms than the political norm, they will all have trimmed to the centre-right to get elected. Even so, it is cheering that some of the traditional negatives seem to be receding. It is, for example, almost impossible to devise a Democrat-Republican stand-off featuring two candidates who will be able to wave happy single marriages at each other.
For years, the belief had been that only a divorcee with a long-distant and discreet first wife (such as Ronald Reagan) was electable in America. But the state of a candidate's marriage faltered as a campaign issue in 1996, when the Republicans felt unable to speculate about the Clinton bedroom because their own man - Dole - had changed wives in dirty circumstances.
The problem for Hillary is that she's a candidate partnered by a former president who ranks among the world's most famous adulterers. This means that her opponents can endlessly invoke "issues" involving her "husband" without being accused of invading her privacy. For this reason, her campaign will pray she ends up running against a Republican divorcee - Giuliani or John McCain - who would be disqualified from chucking love letters.
There's little way of knowing the extent to which race remains a factor in American politics because only voters wearing pointy white hoods are likely to answer pollsters honestly on this. Barack Obama, though, will presumably have thought hard about two factors: that, only two elections back, Colin Powell declined to run partly because he feared a redneck nut with a gun; and that, in current polls, Mrs Clinton is more popular with African-American voters than he is. There is a cruel equation, also observable in showbusiness, that the more appealing black performers become to white folk, the more they risk alienating their own race. And from white rivals and opponents, Obama can be expected to endure euphemistic smears about "background", "judgment" and "character": all code to racist voters to double-take his face.
Drugs, long the second biggest negative after sex on any chief of staff's fear list, are also clearly in flux as a trip-up risk. David Cameron - who is already seeking to overcome the modern prejudice that an Old Etonian will never again occupy No 10 - will also, after this week's leaks from a forthcoming biography, be the first prime ministerial candidate to have admitted illegal drug use in his youth.
The volatility of the narcotics question in British politics is demonstrated by the fact that at least some of the papers publishing young Cameron's scandal were doing so not in order to damage him (as would certainly have been the case 10 years ago) but as part of a campaign to soften cannabis laws. As with the marriage question in US politics, the decreasing danger of pot allegations to a candidate is largely a result of the domination of the electorate by baby boomers, a generation familiar with social spliffs and domestic splits.
Apart from the other distinctions he represents, Obama is also the first major American candidate to have admitted to smoking and inhaling dope at college - although, bizarrely, his fledgling campaign stumbled when it was rumoured that he still smokes ordinary non-waccy baccy. He immediately claimed to be in nicotine rehab. As Cameron also publicly renounced cigarettes shortly after becoming Tory leader, it really seems that fags rather than drugs (or even fagging, at Eton) have become the significant negative today.
Any progress, though, is highly relative. An openly gay president or prime minister remains an outlandish possibility. And the subtlety of these judgments is shown by the fact that for the Democrats to select Hillary Clinton would represent both a breakthrough and a barrier. She would please progressives by being the first ever female nominee, but would continue a sequence in which - in a huge, multicultural democracy - a ticket in every election for 28 years would have contained a member of at least one of two families: the Bushes or the Clintons.
The current range of American candidates, however, highlights the narrowness of the likely next British field. After Conservative experiments with a female candidate (Thatcher) and a Jewish one (Howard), the main-party card at any election contested in the near future would probably contain three white male Protestants: Brown, Cameron, Campbell.
While Thatcher may have been an imperfect advertisement for varying the gender at the top, it must be regarded as depressing that, 17 years after her fall, there is almost no prospect of a British prime minister who departs from the demographic orthodoxies of all recent premiers. A UK equivalent of Barack Obama is very hard to spot - David Lammy, perhaps, but he is nowhere near any leadership betting.
History suggests that the Cameron-Obama scenario that began this piece also remains a long shot, for electoral reasons far more brutal than the kind of cigarettes they smoked. But another double-Wasp male handshake - choose two from Brown, Cameron, McCain or Senator John Edwards - would be a reminder of how closed the arena of political leadership remains.
However much attitudes may have altered to a candidate's use of marijuana or tobacco, the kind of leader who is likely to emerge from the smoke-filled rooms of politics has not begun to change enough.
Mark Lawson