A few years ago, at one of those innumerable Washington occasions where chummy journalists and politicians come together to celebrate each other’s ineffable goodness, the guest of honour, some superannuated media favourite, was being ribbed about how long he had been in public life.

“He’s been in Washington so long,” said the after-dinner speaker, “that he can remember when Joe Biden didn’t have hair.”

It was, of course, a quip about the Delaware senator’s now famous and highly successful hair transplant that transformed him from a prematurely balding fortysomething to the white-mane-flowing senatorial grandee he has become. But it was also, more subtly, a reminder of just how long Senator Biden himself has been around, a signal of what an immovable fixture on the Washington scene he represents.

There are many things that Barack Obama’s new vice-presidential running-mate stands for – working-class values and lifestyle, exemplified by a daily commute to Washington from his plain home in Delaware; a depth of experience in foreign policy and legal matters; a serious, unrelenting and impassioned dedication to the sound of his own voice. But change is certainly not one of them.

He has been in the US senate since Richard Nixon was President. He ran for president himself in 1988 when Senator Obama was still in law school. There is probably no one in Washington better skilled at navigating the headstreams and tributaries of American political power.

Which is why his selection as the number two on the Democratic ticket by Senator Obama, who built his primary campaign attacking the entrenched ways of Washington, is so significant. It shows just how dramatically the Democratic presidential candidate has been forced to recalibrate his campaign as the race enters its critical phase with the opening of the party’s convention in Denver today.

When Senator Obama began his search for a running-mate two months ago, the word from his supporters was that he was looking for a political soulmate. While some were urging him to pick some greybeard, to balance his own youth and lack of experience, the feeling among many around the presumptive nominee was that this would cut across his message of change. Instead, they were looking keenly at possible candidates similar to him: young, fresh, governors such as Tim Kaine of Virginia, and others untainted by years in the ways of Washington.

But in the past few weeks, as the race against John McCain has tightened, it seems that for Senator Obama caution has become the better part of valour. His vice-presidential choice captures the slightly odd ambivalence that will characterise the Democratic campaign as it begins in earnest in Denver. On the one hand he wants to emphasise the message of change and postpartisan newness that his candidacy represents. But on the other his campaign is nervous about the lack of enthusiasm among traditional working-class Democratic voters and wants to reassure them that he is still on their side.

The party will present a unified front to the electorate this week, despite a few low rumblings from supporters of Hillary Clinton. All the leading characters will rally behind Senator Obama and Senator Biden, and campaign organisers say they will not miss the opportunity, as they did four years ago, to bash the Republicans hard all week – with Senator Biden leading the offensive in the traditional running-mate’s role of attack-dog.

It will end, of course, in a crescendo of adulation for Senator Obama when he gives his acceptance speech in the 78,000-seater football stadium, down the road from the convention centre. That speech will, no doubt, be an eloquent paean to Change and Renewal. But it will come after four days in which voters will have seen a very familiar procession of faces – from Ted Kennedy and both Clintons to Nancy Pelosi, the House Speaker, capped off by Senator Biden, the consummate insider, on Wednesday night.

The hope is that the combination will energise new voters and remind traditional voters that this is the Democratic party of old. The danger is that it looks like a Faustian pact, a transaction in which Senator Obama loses the soul of his candidacy.

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