Few big jobs have been reviled with as much intensity as the vice-presidency of the United States.

Thomas Marshall, Woodrow Wilson's veep, hated the pointless impotence of the post. He compared his role to that of “a man in a cataleptic state. He cannot move. He cannot speak. He suffers no pain and yet he is conscious of all that goes on around him.”

John Adams, the very first man to hold the job, described it as “the most insignificant office that ever the invention of man contrived or his imagination conceived”. But the canny Adams also spotted its potential “As Vice-President, I am nothing” he wrote presciently. “But I may be everything.”

Lyndon Johnson made the same point with characteristically more force. Whenever President John Kennedy came into the room, he told friends, “I felt like a goddamn raven hovering over his shoulder.”

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

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BACKGROUND

• The ascent of Barack Obama, Mr Charisma

• Decay, jobs and guns fall under the spotlight

• Free trade: Clinton/Obama's mistakes

With the presidential primary election nearing its end, the US is entering that quadrennial period when the eyes of the world turn to this critically insignificant job. This week, John McCain, the presumptive Republican nominee, let it be known that he has begun the search for a running mate. It promises to be among the most important ever.

That's not because anyone expects the next vice-president to repeat the oddly exceptional role of the current one. There may have been times when people wished that Dick Cheney was in a cataleptic state, but he has been unusually engaged. It's a fair bet that his pre-eminence in the Bush administration is not going to be repeated. The usual significance of the vice presidential choice is that it is, as Adams foresaw, statistically the surest route to a subsequent presidency. Of the last eight veeps who have sought their party's nomination for the top job, only one has failed - Dan Quayle in 2000. Whoever emerges in the next few months as the vice-presidential candidate of one of the two parties is likely to become president himself at some point in the next 10 years.

In Mr McCain's case the choice could be especially important. He is 72 years old and would be the oldest man ever to become president. Though he likes to roll out his 95-year-old mother at rallies as a kind of talismanic testament to his good fortune in the genetic lottery, the chances that, actuarially speaking, he may not complete two full terms in office are quite high. That means his vice-president must truly be ready to take the reins from day one in the job.

Some of the early names getting attention seem to meet that criterion: Charlie Crist, the popular governor of Florida, Joe Lieberman, Mr McCain's closest ally in the US Senate. But they pose a problem. Mr McCain is still strongly distrusted by many conservatives in the party as a closet leftie and these notable moderates will only serve to reinforce that suspicion.

Mr McCain might try to neutralise those concerns by emulating Ronald Reagan in 1976. In an effort to assuage fears that he was too right-wing, Reagan announced - while the primary campaign was still in full swing - that he would have Senator Richard Schweiker, a moderate Republican, as his number two. Mr Reagan eventually lost the nomination at that year's convention by a hair's breadth to the incumbent Gerald Ford, who went on to lose the general election to Jimmy Carter. (When you think what happened to Mr Reagan's eventual running-mate four years later, the first George Bush, it's worth pondering how so much history shifts on the pivot of the vice-presidential choice: we could now be completing the second generation of Schweiker family presidencies.)

In Mr McCain's case the political objective would be the opposite of Mr Reagan's - to reassure the party that he was sufficiently right wing for them. If this is the goal, the names he might choose include Mark Sanford, the governor of South Carolina, and, perhaps the current favourite, Mitt Romney, the former Massachusetts governor whom Mr McCain defeated for the nomination.

But these wouldn't help Mr McCain with another peculiar challenge he faces. For all his evident qualifications and genuine appeal, his campaign is going to lack a little bit of the glamour of the Democratic nominee. It will be hard for voters not to see him as the Grumpy Old White Man taking on the Hip Young Black Guy. That would argue strongly for a gender or ethnic, or at least youthful, dimension to the choice.

Some see Condoleezza Rice, the first African-American female to be Secretary of State, as the answer to this problem. But that probably won't work either.

Mr McCain is going to have to fight hard to dispel the opposition's highly effective claims that given his strong support for the Iraq war, he represents President Bush's Third Term. Having one of the architects of the war and the face of Bush foreign policy for the last eight years as his running-mate and presumed successor is not going to help. There is also Michael Steele, an attractive African-American former Senate candidate from Maryland, and Bobby Jindal, the sparklingly brilliant Indian-American young governor of Louisiana. But they don't truly overcome Mr McCain's initial challenge - finding someone who can plausibly be said to be ready for the presidency.

So Mr McCain is left looking for a young, experienced, capable conservative Republican, preferably with a particular gender or ethnic appeal. There aren't too many of those about, even in America.

Meanwhile, on the Democratic side, the task ought to be easier. Assuming that Barack Obama wins the party's nomination, then surely he will punch the Dream Ticket? After the closest Democratic primary contest in 40 years, it is widely felt that the party will want to unify behind Mr Obama and his narrowly defeated rival, Hillary Clinton, as his vice president.

Dream on, Democrats. Mr Obama has spent the past six months being told by Bill and Hillary Clinton that he is unelectable, unworthy and unprepared for the presidency. Something tells me he won't want, to use Lyndon Johnson's colourful imagery, that particularly opportunistic pair of ravens hovering over his shoulder in the West Wing.

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