Obama just gave a terrific, honest explanation of his economic agenda.
He didn't deny that he proposes some increased spending, but he explained what that spending will go for: clean energy, education, and (mostly) a universal health insurance system. "I think those are pretty important priorities and I pay for them."
And then he pointed out that his tax reform, unlike McCain's, will both raise revenue (at least present to current policy) and shower most of its benefits on the poor and middle-class. McCain's tax plan, by contrast, showers most of its benefits on the wealthy--while actually causing the government to lose revenue.
Obama may be a bit optimsitic when he says he can literally pay for anything; like all campaigns, his makes some favorable assumptions about how poilcy changes will work out. (And if your baseline is current law, which assumes the Bush tax cuts expire, then his package ends up costing the government money, too.)
But every reliable, independent expert agrees that Obama's plan is far more fiscally responsible than McCain's. The definitive word on this comes from the Tax Policy Center.
Oh, and McCain's charge that American corporations pay too much in taxes? Obama's answer was exactly right: Once you take account of the loopholes and such, the corporate tax burden here is average. Paul Krugman has more detail on that here.
Update: The smart folks at ThinkProgress are live-blogging the debate here.