So how will the Democratic Party become more progressive, more left wing, and more courageous if all the progressive and left wing people leave it?
We have two parties in this country. Don't be picky. More beneath the fold.
Third parties can work great on the scale of towns or sometimes states, or if they are dominated by a single personality (like Nader or Perot). I'm not hating on the Green Party, my friend's mom ran for governor as the Green Party and she would have been a kick-ass governor. I even gave her some cash. But she didn't have a chance in hell of actually winning. The entire point of her campaign was to highlight the deficiencies of the Democratic and Republican candidates and to try to change the system from the outside. Her best-case scenario was to get to debate them and force them to make some promises they didn't want to make.
Now lets imagine if Ned Lamont in Connecticut, or Chuck Pennacchio in Pennsylvania, or any other Democrat facing a more conservative primary contest, had said "Screw the Democratic Party, they don't fight for me! I'm running as an Independent!" Pennacchio probably would have raised $4 in funding and Casey wouldn't really have to distinguish himself at all from Santorum, except by the letter after his name. Lamont (if he runs and I hope he does) wouldn't have a chance in hell in beating Lieberman in an open election.
The Democratic Party is a big tent party. There is room for disagreement. Disagreement and dissent, even within parties, is normal, vital, and democratic. As much as I dislike him, there is room for Joe Lieberman as a Democrat because although I can't think of anything he's good for, I'm sure we agree on some issues. Maybe our tent isn't big enough for Zell Miller, but if you post on dailykos.com, it probably is big enough for you too.
If you truly believe in progressive causes, like the right to privacy/abortion, the right to life of people regardless of their ethnicity, the responsibility to ensure that our grandkids have air to breathe and water to drink, then not only should you remain in the Democratic Party, but YOU MUST. The Democratic Party is the most effective mechanism to effect those changes you wish to see on the world.
It is one thing to, as my friend's mom did, try to change the system from outside in an election that means comparatively little (in the gubernatorial election, the Dems controlled the House and Senate by something like a 6:1 ratio). But this country is on the brink. All it takes is one more terrorist attack, or an attempted assassination (whether real or staged) or some nutcase with a nuke, or a spike in oil prices leading to a recession, and we could be looking at a police state. In that situation, would you rather have a Democratic government, or a Republican one? The choice is yours.
Alot of people have said that they aren't going to give money to anybody who voted for cloture on Alito. Guess what? Grow up. If Ben Nelson faces a serious challenge from some nutcase fundamentalist preacher, you can bet your ass I will give him my support. Critics can cry "but the lesser of two evils is still evil!" all they want, but I will defend our tent!
Mao said politics is war without bloodshed. You don't always get to pick and choose your allies.