In the conservative movement, when there are disaffected groups -- religious conservatives, Tea Party members, etc. -- the disaffected people mobilize their anger and take over the GOP establishment (with a lot of help from wealthy friends like the Koch brothers). So, the GOP moves further and further to the right.
On the Left, disaffected progressives often give up on what they believe to be the "hopelessly compromised" Democratic Party and vote for a third-party candidate, like Nader.
I have progressive acquaintances to my left who tell me, "Oh, you can't trust the Democrats. They'll always betray you. They'll use progressive rhetoric to get your votes, but in the end they'll escalate the wars and vote for Wall Street, Big Insurance, and the other corporate interests."
So, too often progressives flee the Democratic Party, causing it to become more centrist. There is mutual anger and distrust between Obama and the progressives, who feel very unloved and unheard. This year it was the Blue Dog Democrats who lost out in the midterm elections. So maybe the Democratic Party will be revitalized with new energy from the Left.
Locally, in Washington State, I see the division in my legislative district and county organizations, where there is constant conflict between progressives and centrists. In party committees, the upstart progressives often leave in exasperation due to the opposition. This harms both them and the Democratic Party.
The centrist Dems in Washington State call themselves "The Road Kill Caucus" (because they're middle-of-the-road and take heat from both the Left and the Right). See this and this. They often vote with Republicans to block progressive legislation.
So far, I'm not seeing progressives mobilize to take over the Democratic Party in Washington State.