The central problem that Obama, Biden, and in fact any democrat has is this: The elections are being fought with two different set of rules. For the republicans, the rule is engage the base and sway the middle at all costs, no matter what lies need to be told or what promises need to be made. The democrats are under a separate rules condition. They must deal with the blatant lies and avoid calling anyone a liar or getting upset, laughing, or being condescending in the face of the lies being told.
This situation is the reason Obama bombed the first debate. He simply did not know what to do. If he responded by explaining the lies are lies with any semblance of emotion, he ran the risk of becoming the ‘Angry Black Man’. If he laid back, as he did, and tried to show he was above the fray, he becomes a detached leader not used to conflict. The spotlight is on him and how he response to Romney’s obvious BS. The problem is there is no right response under this rulescape. He cannot be himself and tell it like it is. That would seem ‘unpresidential’. Meanwhile, if you go the incredulous route, you wind up with the situation that Biden faced. You state the facts, face down the lies, and wind up being called a buffoon for ‘picking on Ryan’.
Basically, the challenger is the victim. If you do anything to damage the victim, then you, as the big strong leader, are the bad guy. Nobody wants to elect the bad guy–well, except when a Bush is involved…