I'm not the world's biggest fan of Barack Obama. But he did make a good point today when he noted the tension between Hillary Clinton's stated willingness to take him on as her vice presidential running mate and her earlier claims that Obama isn't qualified to be president:
"You all know the okey-dokey, when someone's trying to bamboozle you, when they're trying to hoodwink you?" Obama said to the crowd at the Mississippi University for Women. "You can't say that, 'He's not ready [to be president] on day one unless he's willing to be your vice president, then he's ready on day one.'"
Obviously, if Obama is genuinely not ready to be president, it would be irresponsible for Hillary Clinton to have him as her Veep. After all, the main job of a VP, as one former holder of the office put it, is to "wait around the for president to die" and be ready to succeed him at any time.
The underlying problem goes beyond Hillary Clinton, however. Like her, presidential candidates often have strong incentives to choose VPs who might give them an edge in the general election even though they are poorly qualified for the top job. To paraphrase General George Patton, people get chosen for the vice presidency whom neither God nor the Party intended to be president (I paraphrase from memory; if anyone has the exact quote please send it to me).
Several times in our history, this has led to disaster when a dubious Veep ended up taking the presidency after the president died. The most blatant example was when Andrew Johnson assumed the presidency after Lincoln's assassination and promptly began undermining Reconstruction efforts to provide equal rights for blacks in the South, a policy that may have lost the nation it's best chance to overcome the legacy of slavery and at least partially forestall the rise of Jim Crow segregation. Lincoln selected Johnson as his 1864 running mate because he needed a former Democrat and slave state politician to "balance" the ticket. Had he stuck with incumbent VP Hannibal Hamlin (a Maine Radical Republican), history might well have turned out a lot better than it did. I would argue that the succession of VPs Millard Fillmore (1850), John Tyler (1841), Chester Arthur (1881) and Lyndon Johnson (1963) also caused significant harm, though these cases are more debatable than Andrew Johnson.
There are several alternative ways to ensure that a president who dies or resigns is replaced by a politician from his own party without creating the risk of giving the job to a poorly qualified veep. For example, the Constitution could be amended to allow the next president to be chosen by a supermajority of the members of Congress of the president's party. I won't go through all the possible options here. But I suspect at least some of them are likely to be superior to the substantial risks created by the current system.