Discussion in Brickburner and Feral Scholar centers on this question. Josh asks this pivotal question: "The whole plan: "take back Congress and pull the Dems left". When has that ever worked?"
The problem is that, in this, the fourth and last stage of capitalism, under neoliberalism, capital is becoming consolidated on a global level. In such a situation, the "big money" parties are likely to be controlled by global capital, and not by labor. Politics looks like it is moving to the "right" in such a political climate, but only because the "right" is more susceptible to control by the forces of capital than is the "left." Capital occupied a position far to the "left" of where it is today back, say, in 1968, when Keynesian economics still held some currency with American political elites.
It will be hard to stir up enthusiasm for third parties in an era when everyone expects the Democrats to take over for a faltering Republican Party, but neither party promises anything meaningful. But it has always been hard for third parties in American politics -- not impossible, as the naysayers would have it, but hard. Exceptional circumstances must come together with serious effort to produce something good for third parties. So far we have the latter without the former. That may change as the capitalist system imperils itself more and more.