During the 2008 election, Obama co-opted huge portions of the Left and its infrastructure so that their allegiance became devoted to him and not to any ideas.  Many online political and "news" outlets -- including some liberal political blogs -- discovered that the most reliable way to massively increase traffic was to capitalize on the pro-Obama fervor by turning themselves into pro-Obama cheerleading squads.  Grass-roots activist groups watched their dues-paying membership rolls explode the more they tapped into that same sentiment and turned themselves into Obama-supporting appendages.  Even labor unions and long-standing Beltway advocacy groups reaped substantial benefits by identifying themselves as loyal foot soldiers in the Obama movement.
The major problem now is that these entities -- the ones that ought to be applying pressure on Obama from the Left and opposing him when he moves too far Right -- are now completely boxed in.  They've lost -- or, more accurately, voluntarily relinquished -- their independence.  They know that criticizing -- let alone opposing -- Obama will mean that all those new readers they won last year will leave; that all those new dues-paying members will go join some other, more Obama-supportive organization; that they will prompt intense backlash and anger among the very people -- their members, supporters and readers -- on whom they have come to rely as the source of their support, strength, and numbers.