CNN just published its instant poll of people who watched the debate. Result: 51 percent thought Joe Biden did the better job, while 36 percent sided with Palin. The CBS poll of undecided voters yielded similar numbers: 46 percent thought Biden won, while 21 picked Palin.

Let’s stipulate that these instant polls are not the most accurate measures of public opinion. Here’s the interesting thing: The results are virtually identical to the results from last week’s debate between Barack Obama and John McCain. In CNN’s poll, 51 percent thought Obama won while 38 percent thought McCain won.  CBS had it 39-25 for Obama.

Maybe there’s a lesson here. We’re used to analyzing debates based on the candidates’ performance: Were they likeable? Did they make gaffes? And sometimes, surely, that’s the real significance. If George H.W. Bush hadn’t looked at his watch in 1992 or if Al Gore could have kept his sighs to himself, history might be very different.

But for all of the outsized importance character and personality have taken on during this year’s campaign, perhaps the debates will prove significant for a much simpler reason: They conveyed to the voters the philosophy and proposals that each presidential ticket endorses.

Think about it: Obama and Biden don’t really have similar personalities or debating styles; their backgrounds are pretty different, too. The Republican ticket is even more disjointed. McCain and Palin are as different as two candidates can be. But when paired off against their counterparts, both the presidential and vice presidential candidates performed at basically the same level.

It could be coincidence. Or it could be the fact that they made the same essential arguments--and the viewers reached the same conclusions about them.

On economic issues, both Obama and Biden laid out ambitious plans to promote economic security for the poor and middle class: Spending more on education, moving tax breaks from the rich to the non-rich, making health insurance available to everybody. McCain and Palin, by contrast, committed themselves to less spending and lower taxes.

On foreign policy, Obama and Biden attacked McCain for supporting the war and endorsing the doctrine of preemption. McCain and Palin, by contrast, attacked Obama for opposing the surge and proposing engagement with some would-be adversaries. These are pretty clear contrasts on the issues at the top of the agenda.

My reading of the issue polls this campaign season suggest that, in general, voters prefer the Obama approach on both sets of issues to the McCain approach. If so, maybe that's what's driving these decisions over who "won" the debates. 

--Jonathan Cohn