John Nance Garner, who held the position of vice-president under Franklin Roosevelt, memorably dismissed the job as “not worth a pitcher of warm spit”.

Actually, that is the sanitised version of his comment. There are plenty of people who believe that the original version referred to a different bodily liquid. Put it this way: he might have been taking the spit out of this office.

John McCain, by contrast, has to regard it far more seriously. He is within a few days of becoming the de facto nominee of the Republican Party. Mike Huckabee is enjoying his swansong, but after the primaries in Maryland and Virginia tomorrow this contest will be over.

The Democratic dogfight will continue for some weeks yet, with the calendar favouring Barack Obama throughout February then Hillary Clinton in March and probably April. If Democrats are lucky, this race will be settled in Ohio and Texas on March 4. If those states split in their verdicts, then Pennsylvania on April 22 will serve as the Gettysburg.

While all that is going on, the one matter of interest on the Republican side is whom Mr McCain will opt to put forward as vice-president. Ladbrokes has already opened a book on the subject, with Mr Huckabee installed as favourite and Charlie Crist, the Governor of Florida, and Tim Pawlenty, Governor of Minnesota, next. The competition, though, is totally open. No one has a compelling claim.

It is, however, an even more important decision for Mr McCain than usual for presidential candidates for three reasons.

The first is that George W. Bush, via Dick Cheney, has revolutionised the post itself. To be US vice-president was, as Nance Garner implied, to have the largest non-job on the planet. Even when the present President's father was VP under Ronald Reagan, it consisted mostly of attending the funerals of foreign dignitaries (Bush Sr quipped that “you die, I fly” was the vice-presidential motto).

When Dan Quayle was VP he virtually had to beg the White House to provide him with chores to do (this was wisely resisted). Mr Cheney, on the other hand, has shown that the vice-president can be the deputy president and has acted accordingly. It is hard to conceive that this portfolio will retreat to irrelevance again.

Secondly, to put it bluntly, there is Mr McCain's age. He will be 72 come polling day. The chance that he might die in office is there and will be discussed. Whoever he selects to be a potential VP has to be perceived as capable of serving as commander- in-chief at a moment's notice.

Finally, there is the politics of this election. The Democrats have the stardust factor this November. Mrs Clinton and Mr Obama have become huge figures. Their battle will make them seem yet bigger as it intensifies.

If she wins (still the more likely result in my view), Mr Obama will have done well enough to compel her to offer him the No 2 slot in order to preserve party unity (despite their obvious personal animosity and the fact that it does not make much strategic sense) and I suspect that he will accept it. If he wins, the reverse is less likely; but if it is not Mrs Clinton then the temptation to put another woman senator or governor on the ticket will be vast. This is what Mr McCain must assume that he will be facing.

An old-fashioned, tactical vice-presidential pick will not therefore be sufficient. Alighting on a man who rejects the theory of evolution (Mr Huckabee) will not do, nor will taking a Mr Crist or Mr Pawlenty, just because they might help to carry their home state. Seeking to offset a Clinton-Obama duo with either an obscure female such as Sarah Palin (Governor of Alaska) or a black former congressman (J.C.Watts) would seem like a feeble imitation of the genuine item. Condoleezza Rice, meanwhile, has the asset of being female and black but the liability of being tied to the Bush regime.

Mr McCain, who has the advantage of being able to wait until after the Democratic convention before making his move, should be audacious, bold and reach for a man who will reinforce his assertion that national security is the central theme in this election.

That audacious, bold, reinforcing choice would be to nominate General David Petraeus, commanding general of the multinational force in Iraq and the author of the “surge” that has saved the United States in Iraq as well as the Iraqi people (and revived Mr McCain's bid for the presidency in the process). His scheme is now being duplicated successfully in Afghanistan by his disciples in the US Army.

America has a long tradition of looking to military leaders in times of turmoil. This has stretched through Washington to Grant to Eisenhower and might have placed Colin Powell in the Oval Office in 1996 if he had been prepared to stand. General Petraeus, who holds a doctorate from Princeton University, is the greatest military thinker of his generation. He has managed to take a vast army that was effective at conventional fighting but close to useless when confronted with a guerrilla enemy and turn it into an organisation that can today do counter-insurgency superbly. This is an achievement that makes turning a supertanker around on the high seas during inclement weather look as easy as clicking one's fingers. General Petraeus is a genius.

A McCain-Petraeus combination would be a team almost above politics. It would be sensational. It would win. I concede that it is unlikely to materialise. Yet if it did, it would be worth a lot more than a pitcher of warm, er, spit.

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