Presumptive U.S. Democratic presidential nominee Barack Obama introduced his running mate, Sen. Joe Biden, as a potential president with foreign policy heft.

Standing on the steps of the Old State Capital in Springfield, Ill. on Saturday, Obama said Biden was "what many others pretend to be -- a statesman with sound judgment who doesn't have to hide behind bluster to keep America strong."

Biden addressed mainly domestic themes in his remarks to the crowd, assembled in the place where Obama launched his candidacy in January 2007.

Noting that many Americans worry around their kitchen tables about how to make ends meet, he said that Republican presumptive nominee Sen. John McCain has to "figure out which of his seven kitchen tables to sit at."

Earlier this week, McCain stumbled when he couldn't tell reporters how many homes he owned.

Obama also took a partisan shot at McCain, saying the country couldn't afford four more years of the failed "Bush-McCain foreign policy."

Obama welcomed Biden to the stage with the sounds of Bruce Springsteen's "The Rising." When Biden finished speaking and their wives came on stage, the speakers blared U2's "Beautiful Day."

The news went out early Saturday on Obama's website and via a text message that simply said: Barack has chosen Senator Joe Biden to be our VP nominee."

Biden himself has twice sought the Democratic presidential nomination, including this election year.

The 65-year-old has represented Delaware in the Senate for more than 30 years; however, Obama noted that Biden never moved to Washington, instead commuting from his home in Wilmington. Biden brings a depth of experience, particularly in foreign policy and defence issues, to the ticket. Obama has only been a federal senator since 2004.

McCain, an Arizona senator, has cast himself as the national security candidate and has a record of military service dating back to the Vietnam War era. Polls indicate Obama matches up poorly with McCain on foreign policy and defence issues.

Biden also has working-class roots, which is a demographic group that has been slow to warm to Obama. His strength comes from African-Americans, the young and the affluent.

"I can tell you Joe Biden gets it," Obama said. "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 hall in Concord, and at the centre of an international crisis," he said.

News of the appointment leaked to the media well before Obama's aides were to send out the text message. As a result, people on the East Coast got the news at the inconvenient time of 3 a.m.

Attacks begin

McCain's campaign team sprung quickly into action using Biden's own words to attack Obama.

It released a statement saying Biden had "denounced Barack Obama's poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing -- that Barack Obama is not ready to be president."

The Republicans cited an ABC interview clip from August 2007. Biden said at the time that he would stand by an earlier statement that Obama was not ready to serve as president.

Biden's presidential bid faltered in the Iowa caucuses in January. In media interviews before that happened, he didn't appear interested in being a running mate.

"I am not running for vice president," he said in a Fox interview. "I would not accept it if anyone offered it to me. The fact of the matter is I'd rather stay as chairman of the Foreign Relations Committee than be vice president."

In an awkward phrasing back in January 2007, Biden referred to Obama as "clean."

Biden withdrew from the Democratic presidential race in 1987 after being caught lifting lines from a speech by then British Labour Party leader Neil Kinnock.

However, Biden has become a respected senator. He has presided over confirmation hearings for Supreme Court nominees and has convened hearings to examine the Bush administration's management of the Iraq War.

Clinton, McCain

There had been some speculation that Sen. Hillary Clinton, Obama's tenacious foe for the nomination, would be made the vice-presidential nominee. She would bring strength among women and blue-collar voters.

Judge said in his opinion, she would have never been a serious candidate for vice-president.

"The bad blood between the two, the role of former President Clinton in the campaign, the concern that the Clintons would be very difficult if Sen. Obama becomes president -- I think those are all factors," he said.

McCain also must name a running mate. His party's convention begins Sept. 1 in St. Paul, Minn.

Judge said former Massachusetts governor Mitt Romney, a strong rival of McCain's in the primaries, is one name being bandied about as a possible running mate. Another is Gov. Tim Pawlenty of Minnesota, who has less of a national profile.

Judge said Pawlenty is young, has appeal to all groups within the party and could increase McCain's competitiveness in some swing states.

Longshot candidates include:

  • Former Homeland Security czar Tom Ridge
  • Sen. Joe Lieberman, the Independent from Connecticut and vice-presidential candidate for Al Gore in 2000

Lieberman broke with the Democrats over the Iraq War and failed to secure his party's nomination in the 2006 senate election. However, he is listed in the Senate as an Independent Democrat.

With files from The Associated Press