American politics does not want for forward planing. Although the Democratic convention will not take place until the end of August and its Republican equivalent will not occur until the outset of September, speculation has already begun on whom Barack Obama and John McCain will select to serve as their vice-presidential candidates. In Mr Obama's case this is a matter that has to be handled carefully as he has yet to wrap up the nomination formally. Mr McCain, whose campaign has been a little starved of attention while the Democratic duel continues, is probably grateful for the publicity this has generated. He has fuelled it over the holiday weekend by meeting three of the contenders, Governor Charlie Crist of Florida, Governor Bobby Jindal (who is of South Asian extraction) from Louisiana and ex-Governor Mitt Romney of Massachusetts.

Mr Obama's dilemma is straightforward. How can he avoid being stampeded into inviting Hillary Clinton to run with him? That is what much of the Democratic Party wants and it is widely suspected that the Clinton family would accept the role as a consolation prize. While this would secure unity, Mr Obama has plenty of reasons to be wary of the suggestion. The bad blood shed in recent months cannot just be washed away that easily. Having Mrs Clinton on board would seriously compromise the “change” message that has taken him this far and with which he hopes he can travel all the way to the Oval Office. If he won, would he really want her (and him, the husband) present in the same building? If it is not to be Obama-Clinton, would he do better with another woman, such as Governor Kathleen Sebelius of Kansas, or a Clinton supporter, such as Governor Ted Strickland of Ohio?

Mr McCain's conundrum is yet more complicated. His ideal sidekick would be younger than him and manifestly prepared to be the president from the first day (defusing the age issue), have a domestic policy background (the Senator has all the national security kudos he needs himself), be acceptable to conservatives in his party's ranks, ideally offer some diversity of race or gender and be a person whom Mr McCain finds amiable.

It is doubtful whether there is any Republican in the United States who fits all these criteria. Mr Crist, for example, would be a useful ally in winning his home state of Florida, but would he be deemed sufficiently presidential by the media? Mr Jindal's Indian heritage would be a plus but he is 36 years old, only 12 months over the constitutionally required minimum, and half the age of the presidential contender. Mr Romney is probably qualified to serve as the commander-in-chief but millionaire Mormons are not exactly what diversity demands in 2008 and it was evident from the primary contest that Mr McCain could barely stand the sight of him. If a suitable woman senator were available, she would be snapped up in an instant. But those who are available tend to be liberal on abortion, and that would trigger civil war among the Republicans.

So the search is destined to continue for both of these candidates for much longer. And while a good vice-presidential pick is not worth much in votes, a bad one can become an embarrassment. The irony of this situation is that Mr Obama could do with a deputy who looks like Mr McCain, while Mr McCain, in turn, may well obtain most from someone who is similar to Mr Obama.