It would be tempting to assume that, because the political class is obsessed with the vice presidential debate, that Americans will transmute their feelings about the relative debating skills of the vice presidential candidates onto their mental projections of the presidential candidates. Beyond one basic threshold — that of competence — that hasn't happened.
The reason why Joe Biden's debate with Sarah Palin mattered so much last cycle is that Palin had yet to cross the competency threshold. She did not appear to be ready to be on stage with three other national political figures who could plausibly be president. Say what you want to about Paul Ryan, but he is a plausible commander-in-chief. In some sense, then, he has already "won" the debate.
But even implausible candidates, like Dan Quayle, can have bumbling debate performances and still not have much of an effect of the top of the ticket. I was listening to former Dukakis-Bentsen campaign manager Susan Estrich tell Fox News recently how, at the conclusion of that debate, in which Quayle's feathers were clipped, the dial groups showed unanimously that Bentsen had won by every measure. And yet — the bad news — was that the views of the top of the ticket had not changed.
Biden, who is fantastic in these formats when he doesn't have to play nice (as he did, quite obviously, with Palin), should not have the weight of the ticket's renaissance on his shoulders.
The one caveat that occurs to me here is that technology and information consumption patterns have shifted to A-gear so much so that if the political class wants to force the public to make more of a big deal about the vice presidential ticket than they might, then perhaps a rousing debate from Ryan or Biden will shift things.
But I doubt it.