If you look at the Real Clear Politics McCain versus Obama poll compilation chart, you can note that Obama's high water mark prior to the convention was on July 7, 2008. After that, his ratings began a long, steady decline that only stopped, momentarily, for his convention bounce.
What happened?
Well, two weeks earlier, Obama announced that:
"After months of negotiation, the House today passed a compromise that, while far from perfect, is a marked improvement over last year's Protect America Act."
That set in motion a buildup of resentment towards Obama on the left, that reached a crescendo on July 9 when he voted first for cloture against Dodd's filibuster, then voted yes on the Immunity Bill itself.
The result, sadly, was a "perfect storm" designed to erode Obama's to then-growing popularity. First, he turned-off a substantial portion of his liberal base, and I think it took the Convention to regenerate enthusiasm for his candidacy. Second, right or wrong, it was interpreted by hardcore civil libertarians on both the right and left that Obama wasn't going to risk political capital to fight against civil rights abuses. Third, it left Obama open to the "flip flop" meme that is deadly for Democratic Candidates who are always presumed by the media to be less willing to stand on principle when defending America.
Unfortunately, I have no brilliant insight past this observation. I don't know how Obama could go back after that 10% of Republican/Independent voters who really fear the intrusion of Big Brother into their lives, and would have been cheered to see Obama laying down a marker that he didn't intend to use all the powers Bush has consolodated in the Presidency. Instead, Obama offered them a mealy-mouthed verbal committment that while he would give Bush extra-Constitutional powers, he wouldn't necessarily use them himself.
I've always blamed Pelosi/Hoyer for allowing this to have come up in the first place. I'm sure that the AT&T financing of their big show in Denver was nice. But sometimes, doing the right thing is its own reward.