Conventional wisdom says the most important requirement for vice presidents is that they be ready to govern the nation at a moment's notice.

Conventional wisdom says the most important requirement for vice presidents is that they be ready to govern the nation at a moment's notice.

Joe Biden passes that test with ease, but Barack Obama certainly had other criteria in mind when he picked the Democratic senator from Delaware as his running mate.

As chairman of the Senate Foreign Relations Committee, Biden brings valuable foreign policy expertise to the Democratic presidential ticket. Obama lacks experience in that difficult and complex realm.

Biden has been a senator for nearly 36 years, and there are few members of Congress with a deeper or more practical understanding of how Washington operates. Obama lacks Beltway experience, and Biden fills that gap much in the same way that Lyndon Johnson did for John Kennedy in the 1960 election.

Biden's Pennsylvania roots — his family goes back five generations — could help the Democrats win that pivotal state. As a Roman Catholic, Biden may help strengthen support among Catholic voters.

Biden brings negatives to the ticket, as well. Like many in Congress, he loves to talk and talk and talk. He has a legendary propensity for gaffes, including his reference, in announcing his presidential bid in 2007, to Obama as "the first mainstream African-American who is articulate and bright and clean and a nice-looking guy."

In 1987, Biden's first campaign for the presidency imploded after he admitted using lines, without attribution, from John and Robert Kennedy's speeches. He also admitted that he was caught plagiarizing in law school and had exaggerated his academic record. But that was two decades ago, and Biden has become one of the hardest working and most respected members of Congress.

Some have suggested that the selection of Biden underscores Obama's weaknesses. But that's a cynical perspective, one that overlooks Obama's confident and commendable willingness to choose a running mate who brings a wealth of experience to the ticket and who stands ready to occupy the Oval Office at a moment's notice.