Well known blogger Matthew Yglesias (hope I spelled that right) made an interesting point about the fall in influence of "neoliberals," which seems to be defined somewhat as more centrist liberals. It doesn't matter though, his point works for any group. I think what he is saying is that as a tendency acquires some amount of success, that becomes the norm, and what then defines the political tendency is what differs from the norm, i.e. something that was not a chief aim of the tendency in the first place. The liberal example is that as what would now be considered centrist liberals achieved success in some of their goals, they then became the norm, and the democratic party instead became defined by people with different goals. A party is not going to be static. They had their victories, leaving the people who favored them with less to work towards, allowing those with major goals yet to come to fruition (the farther left, in this case) to rise to define the party through interest and activism.

Did that make any sense?