Obama's got a good sense of timing. He (along with David Axlerod) knows when to go positive, when to go negative, when to ignore stuff, and when to push back.
He's also got enough money and and a disciplined enough campaign to do all three simultaneously, while only shifting the elements of his campaign that he so chooses into the national spotlight at any given moment.
Obama's closing speech was good, but at this point in the campaign, I think it's worth revisiting this speech:
I hope that on inauguration day we get another taste of that one.