Precisely. To understand electoral politics in the USA, we have to look beyond party labels. On most issues that manage to get voted on at all in Congress,* the fight is not between the "Democratic Party" and the "Republican Party" but between a majority of Democrats and the _real ruling coalition party_ (Republicans + aisle-crossing Democrats).
Then, there are issues that do not get voted on in Congress, like withdrawing all troops from Iraq and Afghanistan and those on which there exists near unanimity among legislators, like sanctioning Iran. Those undebatable issues are the ones that make America what it is: empire.
Those who are to the Left of the Democratic Party do not have what it takes to dislodge the real ruling party, through a third party or the Democratic Party, electorally. The main reasons are that electoral politics, American-style, is the most expensive form of politics in history and, more importantly, that we are "organizing alone" (to use Jeffrey Fischer's words: <http://mailman.lbo-talk.org/pipermail/lbo-talk/Week-of-Mon-20070806/014592.html>), the apparent national character of America that will not change any time soon.
We have to see what is to be done under these peculiar and difficult circumstances, by taking stock of what strengths actually exist on our side and what can be made of them.
* E.g., the Military Commissions Act, <http://projects.washingtonpost.com/congress/109/house/2/votes/491/>. -- Yoshie