In my travels through the blogosphere, I've encountered lots of people who have some interest in third parties. This includes jedmunds (wherever he is) and some people at Shakes' place -- though perhaps not Shakes herself. Especially after the exciting events of this week, I'd like to offer them some advice and an invitation.
Any movement big enough to make a third-party candidate remotely competitive in a general election can easily win a Democratic primary. This is mostly because there are far fewer Democratic primary voters. 283,055 people voted in Tuesday's primary; 1,253,571 people voted in Lieberman's 2000 general election victory. Democratic primary voters, furthermore, are more accepting of left-wing views than general election voters are. So if you have a movement that's big enough to be competitive in a general election, you can easily swing through the Democratic primary and pick up the nomination on your way. When the general election comes, you'll get the votes of all the straight-ticket Democratic voters, and you'll be the clear choice for every left-of-center voter. It's a lot easier to take over the Democratic Party and win elections that way than it is to build a successful third party.