I understand your point that progressives should run in the Democratic primary. My question for you is: at what point do differences on issues become so great that the entire Democratic Party just doesn't fit your views anymore? There is a difference between moving a party in a particular direction and expecting a complete transformation.
My idea of a political party is an institution that represents a particular political philosophy. One big benefit of having a party at all is that you know where a candidate from that party will likely stand on a variety of issues without having to know anything about the individual candidate. You can vote the party line and know that you are voting for people with similar political positions.
On this score, the Democratic party (as a whole, I understand there are individual exceptions) just doesn't represent progressives or people on the radical left. My perception is that progressives don't stand a chance of winning in the democratic primaries any more than 3rd party candidates have a chance to win in the general election.
Besides, there is the well established tendency already of the major candidates moving to the center after the primaries are completed (Dubya panders to christian conservatives in S.C. during the primaries, but will most likely keep them at arm's length during the general election). When Jesse Jackson ran in the Dem primaries we simply ended up with Republican administraions. I don't really know if this is was due to the inherent weaknesses of the final Dem nominees or whether this actually hurt them in the generals by forcing them left, but the fact is we got Reagan and Bush when all was said and done.
I'd just as soon vote for someone I agree with. I don't care if they can't win. My vote is supposed to be the mechanism through which my political opinions are registered by "the system," and voting for Gore just seems seems like a sellout of my principles. So I'm going to vote for Nader or David McReynolds.
Brett