join them:
1) A fundamental misconception of what the party stands for (Such as the Greens are assumed to be a one-plank party, when this is not the case.)
2) Fear that if they don't vote for the Dems which is the largest party closest to their ideals then their vote will not count.
Many third parties are very active in the lower echelons of politics. You'll find a lot of Greens on school boards, city councils, and managing water districts, for example. But under our current election system it is nearly impossible to win at the federal level.
We need to revamp the system in order for it to work better, both for third parties and for the Dems. (As you can see redistricting the EC votes out of the North into the South hurts them, as does black box voting.)
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FIGHT! Take this country back one town and state at a time!
http://www.geocities.com/greenpartyvoter/electionreform.htm