What if the dynamics of American culture mean that it can only support one major activist movement?
Journalistic narratives cover "both sides", which seems to boil down to those who agree with XYZ and those who don't. But it's possible to disagree with XYZ for both Left-y and Right-y reasons. The biggest win of the Tea Party (or even the Right, for the past 30 years) might then be not the actual force of their argument, but instead that they have successfully stuck themselves into the default counter-position, originally held by (often academic) critics from the left.
In other words, to overcome the TP would first require a single dislodging event that stopped the TP from being the default go-to for don't-agree-with-XYZ. Only once they've been thus-bumped would it be possible for the Left to make any headway.
Maybe OWS could be that one event? Is there some way the Left could regain its place as the default counter-position?