This article
http://www.iht.com/articles/2006/08/09/opinion/ednoam.php contends that a new kind of liberal activist (termed in the article the 'counter-Bushes') is reshaping the rules of US politics. It says that traditionally, Democratic politicians have been supported by interest groups, such as organised labour, and that they have been afforded a wide latitude ideologically so long as their voting record has satisfied their base.
Supposedly, the 'counter-Bushes' are more driven by ideology, and their primary requirement of a politician is that they take the fight to the Republicans. Satisfying the traditional interest groups does not insulate one from the repercussions of an incorrect political ideology, as illustrated by Senator Lieberman losing his Democratic nomination.
Speaking as a relatively disinterested observer - I am british - I would say this seems fairly accurate, except that the article missed out an important detail. American politics has been rewritten, but it is not the liberal left that is doing it. The right already did it. A new breed of rightwingers are in control for whom politics is like warfare. For them winning elections is not enough; the objective is to destroy your opponent. This is why all policy-making in the administration is subservient to the political wing - nobody in the White House is interested in mere governance.
I am far from the action, but it seems to me as if this new kind of politics emerged in the 90s with the Gingrich revolution. The traditional get-along kind of politics does not work against an opposition whose objective is your obliteration. Much of the Democratic leadership has not properly understood this yet, although Lieberman's experience must be providing a salutary lesson. The 'counter-Bush' movement, typified by fund-raising and advocacy groups such as MoveOn and Democracy for America, did not spring spontaneously into being. It is a reaction to the victory of the Gingrich revolution. They understand that consensual politics is impossible with an opposition who considers politics a continuation of war by other means, and that what is required is candidates who will take the fight to the right.