Liberals cannot organize well enough to massively protest for Single-Payer or Government Option Health Care.  Nor for a strong environmental policy, nor for ending the war, nor for Wall Street regulations, nor for marijuana legalization, nor for government accountability over civil rights violations or torture.  Conservatives are crazy and dangerous, but they have gotten their minions OUT to the streets in organized protest campaigns.  And they have muscled their way in from very little positive media coverage before 1977 to basically stipulating the majority of media commentary and debate-framing.  Yes, they had 30 years to do the latter, but they did this despite some utterly disastrous PR for their party (in the name of both Bush presidencies and the tail end of Republican Congressional control in 2003-6.)

In short, the Conservative movement has the fire to actually start a third party.  The only problem is that it would be the de facto Republican party, since virtually every Republican would switch to it.  So there really isn't a need to re-print a bunch of business cards with new logos on them.  The mainstream Republican Party is already on the fringe that the "Tea Party Party" would wish to represent.

The Liberal movement is not even slightly close to being able to break from the Democratic Party.  The urgency is a thousand miles from critical, and in fact could use a mass infusion of the "rabid gene" which poisons the Right.  Or 10 of these infusions.  Anything to move from us calmly stating the facts about health care and global warming and kindly hoping not to get urinated on by the Right as we do this--to move from that to the level of organized message-filtering that Conservatives in the streets with their posters and megaphones and assault rifles use as a matter of course.

We have to start somewhere, yes, and difficult does not mean impossible.  But the hearts of the Progressive movement are not into revolution, much as a few and increasing number of heads may be.