By Jon Frandsen, Senior Editor October 21, 2008 Well, we've all been waiting for Joe Biden to make a huge gaffe and he appears to have done it. Not only did he shine a light on Barack Obama's Achilles' heel, he gave Republican foe John McCain an arrow to aim at it. Biden is so infamous for loving the sound of his own voice and being extraordinarily candid -- as combustible a mixture as you can find in presidential politics -- that it made some think he should be left off the ticket despite his depth of experience, especially on foreign policy, and gifts as a campaigner. Biden doubtless promised Obama he would hold his tongue, and he has avoided causing serious problems at least until a fundraiser in Seattle Sunday night."Mark my words, it will not be six months before the world tests Barack Obama like they did John Kennedy. The world is looking....Remember I said it standing here....We're gonna have an international crisis, a generated crisis, to test the mettle of this guy," Biden said, sounding far more like an opposing candidate than a ticket mate.The results were both predictable and damaging. McCain picked up the line and folded it in into his stump speech. "We face many challenges here at home, and many enemies abroad in this dangerous world," McCain said in Missouri yesterday. "We don't want a president who invites testing from the world at a time when our economy is in crisis and Americans are already fighting in two wars." The gaffe has to be a much needed morale booster for Republicans. Biden managed to uncork the comment the same day that former Secretary of State Colin Powell endorsed Obama in language so strong that conservative icon Newt Gingrich proclaimed that Powell had "eliminated the experience argument" against Obama.Maybe not any more. While Powell's endorsement is still likely to carry some weight, Biden gave McCain a new foothold on the very issue that most troubles voters about the youthful Obama.What was Biden thinking? He wasn't. Whether he's at a committee hearing, an interview or a campaign barbecue, Biden almost always gives into the impulse to show that he's the smartest guy in the room -- which he often is. That's what's so perplexing about Biden. He is an incredibly accomplished politician with encyclopedic knowledge of key issues and a gift for framing those issues in a clear and compelling manner. But he just can't resist talking about what he knows and thinks even if those thoughts aren't particularly helpful to his own goals.Biden also has sharply honed political instincts that generally serve him well -- they just kick in a little too late at just the wrong time. "I probably shouldn't have said all this because it dawned on me that the press is here," he said right after opining on how some misguided nation would be likely to try to take advantage of Obama -- just as the Soviets did with a young John Kennedy when they stuck nuclear-tipped missiles in Cuba. And as further evidence of those instincts, he had sense enough to repeat the remarks later so he could add that those who test Obama will find out he has "a spine of steel."What's worrisome about the incident is that it raises questions about Biden's utility to speak for Obama should they be elected. Sure, Biden could be a helpful and trusted adviser -- and in fact could prove invaluable should his prediction turn out true. But is he going to be someone that Obama would wants out front, speaking for his administration in public, perhaps while explaining policy on a high-profile news show? Could Obama trust him to talk privately with world leaders or to ambassadors, or would he have to worry that Biden might be just a little candid, just a tad too frank?Campaigns are tests -- and campaigns for vice presidential candidates are a chance to see how an understudy might do if the star can't make it on stage. Biden may not have flunked a final exam, but he sure blew an important quiz.