Barack Obama ended days of speculation about his choice of running mate today, sending out an early morning message to supporters saying he had picked Joe Biden.
Biden, 65, is the man seen as the Democrats' authority on foreign affairs.
The voluble chair of the Senate foreign relations committee will appear with Obama for the first time later today in Springfield, Illinois, where the Democratic candidate began his run for the White House early last year.
The pair will then set off on a lightning tour of battleground states, hoping to build up excitement about the ticket before they arrive at the Democratic convention in Denver, which gets under way on Monday.
The choice of Biden was widely seen today as a sign that the Obama camp considered foreign policy would be an issue in the coming election against the Republican, John McCain, and that Obama could suffer from charges of inexperience.
Unlike earlier vice presidential candidates, Biden can not deliver a key battleground state: he comes from tiny Delaware.
After 35 years as a senator, he cannot easily deliver Obama's message of change and – perhaps most crucially – voted for the Iraq war.
Biden could also be haunted by attacks he made on Obama earlier this year, and his plagiarism of a speech by Neil Kinnock 20 years ago, but his knowledge of national security and foreign policy apparently overrode those considerations.
The Obama camp said as much on its website, with a message reading: "Joe Biden brings extensive foreign policy experience, an impressive record of collaborating across party lines, and a direct approach to getting the job done."
Biden reinforced his image as a wise man on foreign affairs with a trip to Georgia a week ago.
That intervention seems to have made him the firm favourite for vice-president, elevating him from a familiar figure in Washington -where he was a regular guest on television chat shows and think tank forums - to an indispensable counsellor.
Despite Biden's ubiquity in Washington, the Obama extracted the maximum drama with the timing of its announcement, keeping the choice under wraps until late on Friday night.
Then, the other top contenders, Tim Kaine, the governor of Virginia, and the Indiana senator Evan Bayh told reporters they would not be chosen.
The official campaign email announcing Biden as the vice presidential pick did not go out until around 3am.
The timing of the decision allows Obama to go into the Democratic convention on a high, rebounding from weeks of being battered by the McCain campaign as a mere celebrity figure unfit for the White House.
Obama had a few days' grace when McCain acknowledged he could not keep track of how many houses he owned with his beer heiress wife, Cindy.
But the Democrats are mindful that he has been on a slow downward slide in the polls all summer, in part because of the McCain attacks. The choice of Biden, with his long record, could reassure voters about Obama.
Biden brings other gifts as a campaigner. He is combative as well as a smooth talker – a combination that could serve him well in the traditional vice-presidential role as attacker.
He can also claim working class roots and is Catholic, which could help win over two core demographics that have so far been lukewarm to Obama.
He is known for regularly returning by train to his native Delaware rather than spending time in Washington. One of his sons, the attorney general for Delaware, is about to be deployed to Iraq in the state national guard.
Biden could also be an asset in trying to unite the party around Obama after the long and bruising primary battle.
He claims credit for passing legislation against domestic violence and is firm in his support of abortion, factors that might help bring support from female Democrats.
In an obviously co-ordinated effort, Clinton sent out an email early today praising Biden as an "exceptionally strong, experienced leader and devoted public servant".
Biden has personal understanding of what it means to be a young rising star in Washington. He was first elected to the Senate at the age of 29 as a change agent.
Even so, he is not an entirely natural choice for a campaign predicated on change, and which has operated for more than a year as an extremely disciplined machine.
Biden's friendliness to reporters – and his occasional use of strong language – could make for an uneasy fit with the Obama campaign's rigour about preventing leaks and staying on message.
Quick talking has embarrassed Biden in the past. Earlier this year, when he and Obama were rivals in the race for the White House, Biden once described Obama as the "first mainstream African-American who is articulate and bright and clean and a nice-looking guy".
Biden dropped out after the first contest in Iowa, having struggled to raise funds from gain supporters against the combined star power of Obama and Clinton. He remained neutral until the end of the primaries, unlike other candidates.
But in the heat of the contest, Biden regularly attacked Obama for his lack of experience – comments that immediately resurfaced in Republican attack ads today. "I would be honored to run with or against John McCain, because I think the country would be better off," Biden says in the old footage.
Biden also has baggage from his first run for the White House.
In 1988, he was forced to drop out of the race for the Democratic nomination after it was revealed he had plagiarised a famous speech by Kinnock in which he talked about how he had been the first in his family to go to university.
