It's ironic but, in a way, what a lot of progressives actually fell for was the anti-Obama right wing propaganda.
I think many on the left heard what Obama was saying during the campaign and thought, "Wow... This guy is just another establishment centrist. Too bad."
But then they exposed themselves to all the sky-is-falling rhetoric coming out of the right-wing media echo chamber and decided, "Hmmm... These guys seem really up in arms. Maybe they're on to something. Everything Obama is saying now is just establishment cover. If he wins he's going to govern as a radically anti-corporate socialist. Whoopee! Go O!"
That's what happened to me at least.