I've studied a bit about community formation, and there's an article in front of me that talks about something well recognized in community building. Namely, that you must always be bringing in new people, especially into the leadership circle. For the community to be sustaining one must recognize that people come and go making new faces incredibly important for the longterm health of the community.
His point is illustrated by something he recognized while campaigning last fall for Kerry. He noticed Mormon Missionaries and their zeal for converts, and something clicked for him. What the far right has done in politics is to always expand the "points of access" through which they can spread their message. The goal being to bring more and more people into their fold.
It seems the "progressives" are, on the other hand, too obsessed with what's happening inside the Beltway and not on building the base, bringing in more people.