By Kevin Connolly
BBC News, Washington
Joe Biden has a reputation for producing memorable soundbites
No-one embarks on a career in American politics with the ambition of becoming vice-president, any more than athletes train for years in the hope of winning Olympic silver.
But the days when those number two slots on the tickets are finally filled provide some of the key moments in any campaign.
The selection of the vice-presidential nominee is often the first really big public test of judgement that a campaign faces - and the choice of running mate tells us a good deal about how the main candidate sees himself.
The golden rule, of course, is to pick someone who is something you are not.
Ronald Reagan - running as the outsider riding in from California to clean up government - picked George Bush senior, the consummate Washington insider.
When Mr Bush's own turn came to run for the White House he was starting to look a little elderly - so he picked the youthful Dan Quayle as his number two.
It is true that Mr Quayle will largely be remembered for spelling the word "potato" incorrectly on a blackboard during a school visit in front of the world's TV cameras, but at the time it made sense to select him because he balanced the ticket.
On the surface Mr Obama's choice of running-mate looks a bit cautious and a little counter-intuitive.
After all, we now face the curious spectacle of a man who's going to change the way business is done in Washington running alongside a political veteran who's now serving his sixth term in the Senate.
But the Obama camp obviously feels that drawback is outweighed by the two factors which define Joe Biden.
Joe Biden (R) has described Barack Obama as "clean and articulate"
First, he is a seasoned political professional who has been in the Senate since 1972
He has bags of legislative experience and he's majored in foreign affairs - so he covers one of the glaring gaps in the Obama resume.
You can't help wondering if the campaign finally settled on Joe Biden after the recent violence between Russia and Georgia reminded American voters that it's a dangerous and unpredictable world out there - a world that might call for a little experience at the White House.
Second, and perhaps more important, he has managed to acquire all that experience while remaining in the eyes of most Americans a regular guy.
He was born in the gritty town of Scranton, Pennsylvania, and has tales of early hardship that will resonate with the kind of blue-collar democrats who tended to support Hillary Clinton in the primaries - but who have so far failed to flock to the Obama colours.
He still lives in the town of Wilmington, Delaware, which is not far along America's rust-belt from his home town, and he even commutes to Washington by train.
He's a Catholic too, which helps with another constituency of doubters, and he is a witty - if occasionally garrulous - speaker. Expect to see him let loose as the Democratic attack dog, free to savage John McCain and his running-mate with the kind of soundbites that make the news and get remembered.
His characterisation of the standard speech by the Republican candidate, and former mayor of New York, Rudy Giuliani, as "Noun.Verb.9/11" is still the best gag of the campaign by miles.
At a personal level too, Mr Biden has dealt impressively with hardship - his wife and infant daughter were killed in a road accident just after he was elected to the Senate (he was sworn in at the hospital), but he recovered to work in the legislature and to bring up his two sons. He has since re-married.
All of those pluses of course, come with plenty of minuses too - Mr Biden was once described as a "gaffe machine", and while that seems a little harsh, there have been plenty of unfortunate moments scattered throughout the Senator's long career.
There are plenty of awkward moments from earlier in this campaign... when Mr Biden had another go at winning the Democratic nomination.
Most damaging of all perhaps was the extraordinary moment in 1988 when it turned out that a stump speech he'd been using as part of his bid for the Democratic nomination had been lifted from the British politician Neil Kinnock.
What made it particularly bizarre was the personal nature of a part of the speech about being the first member of his family to go to college.
It was a misjudgement which Republicans will argue points to some inner flaw.
And there are plenty of awkward moments from earlier in this campaign too when Mr Biden had another go at winning the Democratic nomination.
He described Mr Obama - rather uncomfortably - as a "clean and articulate" African-American, and when asked about the presidential credentials of the man who was then his rival - and who is now his boss - he said "the presidency is no place for on-the-job training".
A Republican attack ad featuring that quote is already running - expect to see a lot more of it.
However, we shouldn't get too carried away with all this.
After all, there's no evidence that a candidate's choice of running mate has any real bearing on their electoral prospects - not since the Texan Lyndon Johnson delivered the Lone Star State for John F Kennedy in the very tight 1960 campaign.
The Obama camp will have weighed Joe Biden's strengths and weaknesses very carefully against each other and decided on balance that he helps the ticket.
As observers we can be sure that his sharp tongue and eventful past will make a race which is already fascinating seem more interesting still.