Barack Obama introduced Joe Biden as his vice-presidential running mate on Saturday, declaring the veteran senator and foreign policy expert a statesman and a fighter who was “ready to step in and be president”.

Appearing with Mr Obama in front of thousands of supporters at a rally in Springfield, Illinois, Mr Biden gave a fiery speech promoting his new partner’s “American values” and assailing their Republican opponents.

He described John McCain, his Senate colleague, as a friend but devoted much of his speech to attacking the presumptive Republican nominee and linking him to President George W. Bush.

The performance suggested Mr Biden will take a highly combative role in a campaign that is already turning increasingly uncivil, freeing Mr Obama to stick to a more positive message.

As chairman of the Senate foreign relations committee, Mr Biden will bring rich foreign policy expertise to the Democratic ticket and help counter Republican charges that Mr Obama lacks the experience to be commander-in-chief.

His Catholic, blue-collar roots could also be an asset as Mr Obama seeks to win over the working–class white voters he struggled to attract in the Democratic primaries.

Delaware Senator Joseph Biden

Mr Obama described his new running mate as a “scrappy kid from Scranton [an industrial town in Pennsylvania]”, who was dedicated to his family and committed to his faith.

”He’s that unique public servant who is at home in a bar in Cedar Rapids and the corridors of the Capitol, in the VFW [Veterans of Foreign Wars] hall in Concord, and at the center of an international crisis,” he said.

The pair spoke on the steps of old Illinois state capitol building, where Mr Obama launched his presidential campaign last year, before embarking on a whistle stop tour of swing states ahead of next week’s Democratic convention in Denver.

Mr Biden, 65, has twice run for president during more than three decades in the Senate – most recently this year, when he dropped out of the Democratic race after a poor showing in the Iowa caucuses.

He was chosen ahead of other contenders including Evan Bayh, senator for Indiana, and Tim Kaine, governor of Virginia.

Hillary Clinton, Mr Obama’s fierce primary rival, did not make the final short list – a snub that threatens to exacerbate tensions between their supporters at next week’s convention.

Mrs Clinton on Saturday praised Mr Biden as a “strong, experienced leader and devoted public servant”. “Senator Biden will be a purposeful and dynamic Vice President who will help Senator Obama both win the Presidency and govern this great country,” she said.

Republican officials said Mr Biden’s foreign policy record only served to highlight Mr Obama’s lack of experience. The McCain campaign immediately launched an advertisement seizing on Mr Biden’s own questioning of Mr Obama’s readiness during the Democratic primaries.

A McCain spokesman claimed Mr Biden had ”denounced Barack Obama’s poor foreign policy judgment and strongly argued in his own words what Americans are quickly realizing that Barack Obama is not ready to be president”.

But Mr Biden made clear during his Springfield speech that he was willing to match the McCain campaign blow for blow over the next two-and-a-half months to polling day.

His most pointed jab came when he joked that, while ordinary Americans were sitting around kitchen tables worrying whether they can meet their mortgage payments, Mr McCain’s only concern was “which of his seven kitchen tables to sit at”.

That was a reference to a recent interview during which Mr McCain appeared to forget how many homes he and his wealthy wife, Cindy, owned.

“You cannot change America when your first four years as president will look exactly like the last eight years of George W. Bush’s presidency,” he said, underlining the main Democratic line of attack against Mr McCain.

He also sought to answer Republican questioning of Mr Obama’s patriotism and values and drew parallels between their respective upbringings in hard-pressed families with deep US roots. “Barack and I come from very different places but we share a common story. An American story,” he said, recalling how Mr Obama’s grandfather served under General George Patton in the second world war and how he was raised by a single mother relying on food stamps.

Mr Obama’s choice of running mate was the source of feverish speculation for days but the decision was kept secret until the early hours of Saturday morning, when news organisations started to report that Mr Biden had prevailed.

The leak forced the Obama campaign to accelerate its much-hyped plan to announce the decision direct to supporters through millions of text messages and emails.

Mr Bayh and Mr Kaine had been hotly tipped because their respective home states are both top Obama targets in November. As senator for Delaware, a safely Democratic state, Mr Biden offered a different kind of appeal.

He emerged among the leading contenders relatively late in the selection process as Mr John McCain, stepped up his attacks on Mr Obama’s foreign policy credentials.

Foreign policy and national security have been overshadowed by economic concerns for most of this year but the crisis in Georgia has thrust international affairs back to the centre of the campaign.

Mr Biden recently returned from a trip to Tbilisi for talks with Mikheil Saakashvili, Georgia’s president, highlighting the foreign policy gravitas he could bring to the Democratic ticket.

However, his selection also brings risks.

The choice of a long-time Washington insider as running mate could undermine Mr Obama’s promise to bring a new kind of politics to the capital. Mr Biden’s reputation as a brash and long-winded orator, meanwhile, creates potential for damaging gaffes.

During the primaries, he caused uproar by praising Mr Obama as a “bright and clean”, apparently drawing a contrast with other African-American politicians. Mr Obama said he was not offended by the remark.

His first run for president in 1988 ended amid controversy when he was found to have plagiarised a speech by Neil Kinnock, then-leader of the British Labour party.

In addition to chairing the Senate foreign relations committee, he has also headed the body’s powerful judicial committee, which oversees judicial nominations and constitutional issues. He was elected to the Senate at age 29 in 1972.

His first wife and 13 month-old daughter were killed in a road accident shortly before he took office and he has twice survived brain aneurysms.

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