Joe Biden: The ideal foil to the president
Joseph Biden's foreign policy expertise and blue-collar appeal made him an astute addition to Barack Obama's ticket in 2008. And in the years since he was elected to that role, he has been a trusted adviser - and occasional pain in the neck - to the president.
A vice-president's first job is to get the ticket elected. And Mr Biden was chosen for the 2008 Democratic ticket to fill two gaps in Mr Obama's political repertoire.
Mr Biden, a veteran Delaware senator, was selected for his supposed folksy appeal to white working-class and middle-class voters.
And as a long-standing senior member of the Senate foreign relations committee, he could defend the ticket against Republican criticism that Mr Obama was ill-equipped for the role of commander-in-chief.
The two men complement each other in several other ways. While Mr Obama comes across as cool and collected, Mr Biden can be lively and combative in his attacks on opponents.
Mr Biden also relishes the old-fashioned politics of glad-handing and party networking that Mr Obama appears to dislike.
In the years since the 2008 election, Mr Biden has served as a trusted adviser to the president.
He has Mr Obama's permission to attend any West Wing meeting he wants, and has the final whisper in the president's ear on major policy decisions, according to a recent profile in New York Magazine.
But Mr Biden has also earned a reputation as a loose-lipped gaffe machine, offering fodder to the conservative media, distracting the cable news nets and knocking the White House off message.Triumph and tragedy
Joseph Robinette Biden Jr was born in 1942 in Scranton, Pennsylvania, one of four children in an Irish-Catholic family. In American political code, hailing from that blue-collar town gives a politician credibility with middle America, and not least with the news media.
The family later moved to the small north-east state of Delaware, where Mr Biden attended the University of Delaware. He went to law school at Syracuse University in New York state.
He was elected to the US Senate in 1972 at age 29, and took office weeks later aged 30, the minimum age to enter the Senate.
But before he could take office, his family was devastated by tragedy. His wife and infant daughter were killed in a winter car accident.
At a recent 9/11 commemoration, Mr Biden spoke of his pain.
"No matter how many anniversaries you experience," he said, "for at least an instant, the terror of that moment returns; the lingering echo of that phone call; that sense of total disbelief that envelops you, where you feel like you're being sucked into a black hole in the middle of your chest."
Mr Biden ran for the presidency in 1988 but withdrew after he admitted plagiarising a speech by the then leader of the British Labour Party, Neil Kinnock.
He spent the next years rising through the Senate ranks, ultimately becoming chairman of the judiciary and foreign relations committees.
In 2008 he ran again for president but failed to gain traction and dropped out in January.
Mr Biden's reputation as an able politician has been marred by his loose tongue.
In 2007, he described Barack Obama as "the first mainstream African-American who is articulate and bright and clean and a nice-looking guy".
Shortly after the pair took office in early 2009, Mr Biden said in a speech that no matter how hard he and Mr Obama tried to get things right, they still ran a 30% chance of fouling up.
Asked about the remark at a press conference, Mr Obama chuckled.
"I don't remember exactly what Joe was referring to," he said. "Not surprisingly."
In the current campaign, with its hypercharged news cycle, he has been criticising for telling an audience of African Americans that Republicans would put them "back in chains".
Recently, Republicans jumped on his remark that the American middle class had been "buried" in the past four years - while Mr Obama was in the White House.Same-sex marriage
This year, one of Mr Biden's off-message comments significantly altered White House policy.
Mr Biden told an interviewer that he supported same-sex marriage, though Mr Obama had said for years his own position was "evolving".
White House aides were said to be furious, but within days, Mr Obama said he had come around and now supported legalising same sex marriage. The political fall-out has been negligible.
"Thank you for being the best vice-president I could ever hope for," Mr Obama said at the Democratic Convention in September.
In the 2012 campaign, Mr Biden's role has been to keep up the attack on Republican challenger Mitt Romney and his own running mate Paul Ryan, while Mr Obama's aides hope he keeps verbal blunders to a minimum.
If Mr Obama wins in November, Mr Biden is seen as a likely candidate for president in 2016. And given his apparent success as vice-president now, he could find himself at last on top of the Democratic ticket.