SPRINGFIELD - Standing again in front of the antebellum limestone columns where it all started 18 months ago, Barack Obama on Saturday introduced his newly chosen running mate, Sen. Joe Biden, as a "scrappy kid from Scranton" turned world-class statesman, and immediately set him loose on John McCain. | Video | Biden gets good marks from Obama backers | GOP: Biden himself said Obama has no experience | Photo gallery

Biden wasted no time in stepping into his twin roles as partisan attack dog and ambassador to the kinds of Americans who "sit around kitchen tables" struggling with household bills.

"That's not a worry John McCain has to worry about," the long-time Delaware senator told thousands of frenzied supporters in Springfield, referencing recent media reports that McCain was unsure how many houses he owns. "He'll have to worry about which of the seven kitchen tables to sit at."

Obama, the Illinois senator and presumptive Democratic presidential nominee, returned to the site of his unlikely campaign launch in February 2007, the Old State Capitol in Springfield's Lincolnesque downtown plaza, to introduce a running mate who could fill in some of the gaps in Obama's political resume.

Obama's introductory speech lauded the long-time Delaware senator as "one of the finest public servants of our time." He then went through a detailed outline of Biden's life, painting him as a man with a humble, hard-knocks background and sterling foreign policy bona fides.

"(He) has always been a friend to the underdog," said Obama. "He looked (former Serbian leader) Slobodan Milosevic in the eye and called him a war criminal. . . . Joe Biden is what so many others pretend to be: a statesman."

Biden, 65, lauded his 47-year-old freshman Senate colleague as visionary leader.

Biden also slammed Republican presidential candidate McCain as a potential extension of George W. Bush's presidency, stepping quickly in the traditional shotgun seat for vice-presidential candidates.

"We cannot as a nation stand four more years of this," Biden said, to enthusiastic cheers from rally-goers who stood for hours in 90-degree heat to witness the unveiling of the ticket.

Biden also clearly is being brought on board to help connect Obama to skeptical working-class voters.

"Barack and I come from very different places, but we share a common story," said Biden, recounting how Obama was raised by a single mother and how both men were poor at times in their childhoods. "We share the same values."

Even the attire of the two men appeared coordinated: no jackets, just shirts and ties - Obama's solid red, Biden's solid blue - with both sets of shirt sleeves rolled up.

But the careful planning didn't prevent minor gaffes by both men. Obama, in a frenzied build-up introduction at the end of his speech, initially touted Biden as "the next president," before quickly demoting him back to running-mate status. Biden later tripped on Obama's last name, referring to him as "Barack America."

Around the Old State Capitol, an imposing 1830s building where Abraham Lincoln delivered his "House Divided" speech, Obama supporters who first learned of the choice of Biden hours earlier through news reports or text messages from the campaign, appeared to generally supportive.

"Biden is the bulldog we need," said Sharon Hoffmann of St. Louis, who traveled to Springfield for the event.

Not everyone agreed. "Biden had a lot of criticism of Obama in the primary race," noted rally-goer Shane Mills, also of St. Louis. "I think that's going to be an easy target for McCain. I think Obama's got a lot of work to do."

The Springfield event kicked off several days of appearances in battleground states for the two men before they head to Denver for the Democratic National Convention.