I decided years ago that I would vote for the less crazy candidates which have a chance to win, and for a long time those have been the Democratic candidates. But I did not have the heart to work for the conservative Obama, and I do not have the heart to work for a conservative party. This is not the Democratic party of our youth: all the great liberal Democratic Senators of that time are gone and the Democrats are dominated by their conservatives. On a personal level, I think you will find supporting a party which compromises with the anti-abortion right very hard. (And the opening of new coasts to offshore drilling, as well, which hadn't occurred when I wrote the original letter.) I don't see the Democrats as moving to the left in this decade. Maybe if there are more major environmental disasters they will improve their environmental policies.
The Greens were grandstanding in 2000. I hope we never see that again. Any third party has to start at the state and local level, and in the House. On the other hand, the Republicans are a rump party. Their policies are enormously unpopular and, without the arcane rules of the Senate, and its undemocratic electoral structure, they would be very weak. Yet on major environmental issues the conservative Democrats are so far only a little better than the Republicans. At least the Democrats are willing to accept and support science. I like Dr. Stephen Chu--besides, he may fund a job for me. But I don't see the Democratic leadership challenging the oil (called that one!), coal, automobile, or road-building industries, any more than they have challenged the financial services industry or its health insurance branch. Matters are going to have to get much worse before the Senate will act, and I think a challenge is more likely to come from outside the Democrats than within.
Meantime: more women in the House, more women in the Senate, more women on the Federal bench. Electoral reform. Separation of church and state. Health care for everyone. Jobs. It's the environment, stupid!