This article was published before the Biden campaign's announcement that California Senator Kamala Harris would be Joe Biden's running mate. Does the argument still hold up?--Ed. 

Conventional w­­isdom claims that a vice presidential choice doesn’t much matter. Under this stale view, the only modern exception is Lyndon B. Johnson in 1960, who helped John F. Kennedy carry Texas and other crucial Southern states to clinch a close election.

But helping to win a pivotal state is by no means the only measure of a successful vice presidential pick. Presumptive Democratic nominee Joe Biden is doubtless well aware that there are other important ways the candidate for No. 2 can “bring real strength to the ticket,” as Franklin D. Roosevelt put it in a letter endorsing Missouri Sen. Harry S. Truman or Justice William O. Douglas as his running mate in 1944.

For Biden, the first is choosing someone well-suited to be president, if necessary, on a ticket that embodies the glorious diversity of America. (Until now, our vice presidents have included zero women, zero African Americans and exactly one traditional practicing Catholic — Biden.) Here are some other measures:

Provide personal chemistry with the nominee. In the summer of 2004, the Democratic nominee, Sen. John F. Kerry (Mass.) was so queasy about choosing the widely-recommended Sen. John Edwards (N.C.), whom Kerry viewed as insincere and undependable, that he called his Massachusetts colleague Ted Kennedy and said, referring to the senator from Connecticut, who was Kennedy’s close friend, “Make the case for Chris Dodd.”

In the end, Kerry did not choose Dodd, whom Biden has now asked for help in selecting a running mate. But Kerry’s qualms about Edwards were later proved right when Edwards did not live up to his private assurance that he would not run in 2008 if Kerry were considering the race. (Kerry didn’t end up running.)