The only question is when will people dare to take the first steps towards breaking the damaging two-party system that, to be honest, has a massive, almost unbreakable, foundation in America. Most people who would form and support a true and new labour party in the US are located on the left, and those people vote democrat today. So if a new party is to be formed the Democratic party must first die, or at least lose a lot of its current support. And no matter how conflicted and crazy the right-wingers are they are good at one thing: sticking together for a common goal. That would mean that during this transition the Republicans would most likely gain more power. And I guess this is what holding most people off, otherwise the Greens in the US would already be a major player in the national politics.
But then again everything is possible. And I don't think that its just people on the left that wants to break away from the two-party system. The internal fighting within the Republican party between the more traditional republicans, the neocons, and the religious fringe could easily escalate. I could easily imagine that the Tea Party movement, for example, could spring lose from the Republicans and form their own party.
People should stop to assume or even hope that the Democrats will ever take ordinary people's wishes and needs in consideration. The democratic party is just a lesser evil than the Republicans. At the moment the political system in the US boils down to a choice between the plague or cholera.
It will probably be a painful transition, but it's a transition that is very much needed.