The question is whether you have a leader from the Democratic party who's willing to head left like FDR did, or whether you have someone who closes off the possibility for the Democratic party to move Left.  And if the path Left within the two party system is closed off, then the only other option is from a third party rising up to the Left of the Democratic party to push the discussion to the Left.

Maybe I shouldn't be so dismissing of the possibilities for that to happen without prompting conservative backlash.  The rise of Labour from the shadow of the Liberals stands out as a model, but that was prompted by a simultaneous expansion of the franchise, and the removal of power from the House of Lords.  In short, it was the continuation of a process of change set forth from within the system.

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg