I have no inside information and may be wrong, but my best guess is that Joe Biden will be Barack Obama's choice for vice president for some of the very reasons that were thought to disqualify Biden. If it is Biden, the three biggest advantages he brings will be his ostensible shortcomings:

His mouth: Biden would fulfill the job of attack dog that is the first requirement for a vice presidential candidate, and that is especially important now for Obama. If Jabbering Joe is responding to John McCain's shots with memorable one-liners of his own, Obama can stay where he wants to be—above the fray. And if Biden says something off-the-wall that sticks in everyone's mind, all the better, as long as it's about McCain and not Obama or people who work in convenience stores or otherwise loosen Biden's tongue. The worry with Biden is that he just can't help himself. Obama may hope that he just can't stop himself from saying, say, that McCain is a hothead who shouldn't have his finger on the button. Obama can then denounce his No. 2's intemperate remarks even as they sink in. This is what veep candidate were put on earth to do. Same on the Republican side.

His age: Biden is 65 and has been around Washington since 1972. That's supposed to be off-message for Obama, whose theme is change. But people forget that the selection of Dick Cheney in 2000 helped George W. Bush prevail. Voters reasoned that Bush might be green but at least he'd have Cheney around him for sound advice. This logic would be especially helpful to Obama on foreign policy. Biden's experience there won't diminish Obama; it will free him to focus more on the economy. The main task now for Obama is reassurance that he could handle the job, especially commander in chief. Biden provides it.

His state: Biden is from tiny Delaware, but he was born in Pennsylvania and his Catholic background and compelling life story (his wife and baby daughter were killed in a traffic accident, and he took the train home every night for decades to be with his family) will help in several swing states. His son Beau, the attorney general of Delaware, is a captain in the Delaware National Guard and is shipping out to Iraq in September, which doesn't hurt in states with large military populations. And Biden is very popular among Jewish voters, who might be important in Ohio and Pennsylvania, not to mention Florida.

Obama and Biden were not close in the Senate, and Biden, amazingly, has still not formally endorsed him. But even this could be turned into an advantage, as Biden encourages wary supporters of Hillary Clinton to make the journey with him from suspicion of Obama to full embrace.

Editor's Note: Based on further reporting, this column was amended regarding the timing of Obama's veep selection.

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