Barack Obama will today bring an end to a week of frenzied speculation by unveiling Joe Biden as his vice-presidential running mate, a choice that he hopes will help fill some of the gaps in his own resume.

The 65-year-old Senator is expected to appear at a lunchtime rally alongside Mr Obama in Springfield, Illinois, where the Democratic nominee launched his campaign for the presidency 20 months ago.

Mr Obama had succeeded in keeping the identity of his vice-presidential pick secret until late last night when it was reported that a team of Secret Service agents had arrived at Mr Biden's home just hours after other candidates on the shortlist, such as Virginia Governor Tim Kaine and Senator Evan Bayh, were being told they were out of the running.

A text message will be sent out to millions of supporters this morning informing them of the decision, just two days before the Democratic convention gets underway in Denver on Monday.

Mr Biden, the chairman of the Senate Foreign Relations Committee, is a seasoned political operator and steeped in knowledge about international affairs while also being seen as capable of broadening Mr Obama's appeal to the fellow working class Catholic vote in key states this November.

The Delaware senator was born into a blue-collar family and grew up in hard-scrabble Scranton, Pennsylvania. But he still carries risks. Republicans are certain to seize on remarks he made during the Democratic primaries suggesting Mr Obama, having been elected to the Senate for the first time in 2004, was too inexperienced to be president because the job did not allow for "on-the-job training".

Mr Biden, who dropped out of the race after securing just one per cent of the vote in Iowa's caucuses, has a reputation for garrulousness and liking the sound of his own voice. Last year he was forced to apologise to Mr Obama for describing him as “the first mainstream African-American who is articulate and bright and clean and a nice-looking guy.” Another slip in 1987 destroyed Mr Biden's first bid for the presidency when he was caught plagiarising a speech by Neil Kinnock, the then leader of the British Labour Party.

Although the 36 years Mr Biden has spent in the Senate - during which he controversially voted for the Iraq war - might appear to be at odds with Mr Obama's promise of a "new politics" that changes Washington, the Democratic nominee is said to have been impressed by his running mate's straight talking loyalty and ability to "get things done".

Mr Biden also has a compelling personal story: in 1972, his first wife, Neilia, and 13-month-old daughter, Naomi, were killed when a tractor-trailer crashed into her station wagon as she drove home with a family Christmas tree. His two sons, now grown-up, were seriously injured and he cared for them at home for a long time.

In February 1988, Mr Biden survived his own brush with death after suffering two brain aneurysms which kept him from the Senate for seven months.

Hillary Clinton, who ran Mr Obama close for the Democratic nomination, had earlier yesterday appeared to distance herself from speculation that she might yet be picked as his running mate, saying she had never asked him for the job.

Her supporters, however, expressed irritation that she had not been properly considered by Mr Obama's team or even consulted about who should get the job.

Some are already planning to express their unhappiness publicly at the convention next week. "I don't think Senator Obama has had a conversation with Senator Clinton about the vice presidency, either about her, or about seeking her counsel about who it should be," said James Carville, a veteran Democratic strategist from the Clinton White House. "There's no doubt that some people are going to view this as that she is not being accorded respect."

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