Two not so small points.
1. His remarks “The fundamentals of the American economy are strong” delivered in mid September as Lehman Brothers was going up in smoke and the markets were getting pummeled. It showed voters that he was ignorant and out of touch with the information they could see in the crawler at the bottom of their screens as he was uttering that mistake.
2. He famously came in from the campaign trail to return to Washington to personally get the stimulus bill passed. Obama, just as much a Senator as McCain, stayed out on the trail and avoided responsibility for the failure of the stimulus bill to pass. McCain looked foolish and ineffective. And lots of people noticed.
He never recovered from that.