It was a reference to McCain's recent inartful admission - in a time of economic uncertainty - that he was not sure how many homes he owns.
Before a vast crowd spilling out from the front of the Old State Capitol, 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."
Democrats coalesced quickly around the 47-year-old Obama's selection of a seasoned veteran of three decades in the Senate - a choice meant to provide foreign policy heft to the party's ticket for the fall campaign against McCain and the Republicans.
Polls show Obama rates relatively poorly against McCain on foreign policy issues, and Biden is chairman of the Senate Foreign Relations Committee with extensive experience in that area.
McCain called Biden a "wise selection." But McCain indicated that he believed there was still plenty to criticize the Obama-Biden ticket on foreign policy.
"I know that Joe will campaign well for Senator Obama, and so I think he's going to be very formidable," McCain told CBS News. "I've always respected Joe Biden, but I disagreed with him from the time he voted against the first Gulf War to his position where he said you had to break Iraq up into three different countries. We really have different approaches to many national security issues."
Biden, a 65-year-old congressional veteran, emerged as Obama's choice after a secretive selection process that reviewed at least a half-dozen contenders - but evidently not Hillary Rodham Clinton, the former first lady who was Obama's tenacious rival across the primaries and caucuses of winter and spring.
Obama made a symbolic choice for the site of the ticket's first joint appearance.
It was a brutally cold winter day more than a year ago when he stood outside the historic structure in the Illinois capital to launch his quest for the White House.
He returned this day in sunshine, the party's improbable nominee-in-waiting, a black man in his first Senate term who outdistanced a crowded field of far better-known and more experienced rivals for the nomination.
The Democratic National Convention opens on in Denver Monday to nominate him as president and Biden as vice president, the ticket that Democrats hope to ride into the White House after eight years of Republican rule.
Polls indicate a highly competitive race at the end of a summer in which McCain eroded what had been Obama's slender advantage in the national surveys.
A security fence sprung up overnight around the Pepsi Center as the pace of preparations accelerated in advance of Monday night's opening session, and police on bicycles patrolled nearby streets. Inside the sports arena, even the Zamboni machine - the lumbering, wheeled vehicle used to resurface the ice between periods of hockey games - had been moved out to make room for the Democrats.
McCain's convention opens on Labor Day in St. Paul, Minn. He has yet to select a running mate.
Responding to Obama's pick, the McCain campaign quickly produced a television ad featuring Biden's previous praise for McCain and comments critical of his new benefactor. In an ABC interview last year, Biden said he stood by an earlier statement that Obama wasn't yet ready to be president and "the presidency is not something that lends itself to on-the-job training."
In addition, the Republican Party arranged for an independent expenditure advertising campaign to coincide with the Democratic convention. One individual familiar with the plans described a $2.25 million effort on cable and broadcast stations through Aug. 31 in the battleground states of Colorado, Ohio, Wisconsin, Michigan and Pennsylvania.
Obama brought Biden on stage with his glowing introduction to the strains of Bruce Springsteen's "The Rising."
The newly named running mate moved center stage in shirt-sleeves at a brisk trot that belied his 65 years, and embraced Obama. "I'm glad to be here," said the man who has twice sought the presidency. Thousands of newly printed signs bearing the words Obama/Biden sprouted in the crowd that waited in anticipation in 90-degree temperatures.
Both men spoke for 16 minutes - unlikely a coincidence given Biden's reputation for verbosity.
Obama's remarks were carefully crafted to emphasize Biden's accomplishments in the Senate, his blue-collar roots and - above all - his experience on foreign policy.
"I can tell you Joe Biden gets it," he 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 center of an international crisis," he said.
In contrast to the Obamas and the McCains, the Delaware senator isn't a multimillionaire. Biden and his wife, Jill, have $59,000 to $366,000 in assets and $140,000 to $365,000 in debts, including a $15,000 to $50,000 line of credit Biden co-signed with his son to cover college expenses, according to a financial disclosure report for 2007, which describes assets and liabilities in ranges.
Obama recounted the personal tragedy that struck Biden more than 30 years ago, within days of his election to the Senate, when his first wife and their child were killed in an automobile accident.
He said Biden raised his surviving children as a single parent, commuting between the Capitol and Delaware daily on the Amtrak train.
"For decades, he has brought change to Washington, but Washington hasn't changed him," Obama said, attempting to blunt an emerging Republican line of attack that notes Biden's three decades in the polished corridors of the Capitol.
"He's an expert on foreign policy whose heart and values are rooted firmly in the middle class."
In a jab at McCain that foretold Biden's far sharper criticism, Obama said his political partner "will give us some real straight talk."
Biden blended praise for Obama and criticism of McCain. "You can't change America and make things better for our senior citizens when you signed on to Bush's scheme of privatizing Social Security," he said.
"You can't change America and end this war in Iraq when you declare - and again these are John's words - 'No one has supported President Bush in Iraq more than I have,' end of quote."
David Axelrod, Obama's senior strategist, described Obama's vice presidential search as "a long process but it always pointed in Biden's direction."
Sen. Dick Durbin of Illinois, who is close to Obama, added, "Many others were discussed but my impression was that those three a few weeks ago were really the centerpiece - Kaine, Bayh and Biden." He referred to Virginia Gov. Tim Kaine and Indiana Sen. Evan Bayh.
Associated Press Writer Jim Kuhnhenn contributed to this report from Denver.