In choosing a running mate, every presidential candidate insists that he is seeking above all someone who could serve as president at a moment’s notice. Often that assertion is patently preposterous. With his choice of Joe Biden, Barack Obama can make that assurance with a straight face. The six-term senator from Delaware is serious, substantive and sophisticated in his understanding of the world.

Political junkies already have generated reams -- and gigabytes -- of commentary about how Biden will or will not complement Obama in his contest with Republican candidate John McCain. We’re not so naive as to believe that electoral calculations played no part in this choice. But Biden passes the “ready on day one” test better than most vice presidential candidates. Think of Dan Quayle, the telegenic but callow Indiana senator plucked from obscurity by George H.W. Bush to shore up the support of conservatives. Or Geraldine A. Ferraro, the obscure New York congresswoman selected by Walter F. Mondale because he believed a female running mate would energize a listless campaign.

Biden, who sought the 2008 presidential nomination himself, is often caricatured as a self-important blabbermouth. Like most caricatures, that image has some basis in reality. But it is the same Biden who, as chairman of the Judiciary Committee in 1987, effectively -- and economically -- questioned Supreme Court nominee Robert H. Bork. His questioning elucidated Bork’s extreme views in opposition to a constitutional right to privacy, the cornerstone of Supreme Court decisions upholding the right to contraception and abortion. Bork’s nomination was defeated on the Senate floor. Biden, currently the chairman of the Senate Foreign Relations Committee, also is well versed in a range of foreign policy issues. Most recently he has demonstrated a grasp of the intricacies of the political situation in Iraq, although his preferred solution -- a “soft partition” of the country into Sunni, Shiite and Kurdish regions -- never gained traction.

In turning to a familiar Washington figure, Obama has opened himself to two criticisms: that he is compensating for his thin resume, much as George W. Bush did with his choice of Dick Cheney, and that the addition of Biden to the ticket undermines Obama’s argument that wisdom trumps experience. Obama deserves credit for not allowing these tactical objections to keep him from choosing an impressive partner and president-in-waiting. Instead of attacking Obama’s approach to choosing a running mate, McCain should emulate it.