It was a reference to McCain's recent inartful admission 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 Obama's selection of the 65-year-old 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.
Obama made a symbolic choice for 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 47-year-old black man who outdistanced a crowded field of far better-known and more experienced rivals for the nomination.
The Democratic National Convention opens on 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.
McCain's convention opens on Labor Day. He has yet to select a running mate.
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.
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 and emerged as Obama's pick only in the past few days.
Thousands of newly printed signs bearing the words Obama/Biden sprouted in the crowd that waited in anticipation in 90-degree temperatures.
Both Obama and Biden 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.
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 30 years 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."