This is from the Nation editorial linked to above by 999:

Mainstream voices tell progressives unhappy with Obama to grow up: your whining threatens to elect a Republican in 2012, who would be much worse. But they are the ones who aren't savvy. Fear of the dark side will cause most of the Democratic base to give Obama their votes, but it will not be enough to persuade them to give up their evenings and weekends to get out the vote for him, to sway independent and undecided voters. It's a normal reaction. If Obama approves the pipeline, explains Courtney Hight, his Florida youth-vote director in 2008 who was arrested in the protest outside the White House, "it is just human nature that the resulting disappointment will sap the enthusiasm that drove us to work so hard last time."

Obama still has time to bring his message into line with the stirring vision he conveyed in 2008. Perhaps, however, he thinks he can win without a strongly motivated base, relying instead on the powers of incumbency, not least the enormous amounts of money he is raising. If he chooses that course and it fails, spare us any prattle about unsophisticated progressives being at fault. A defeated Obama will have no one to blame but himself.