So basically my question is how does this happen, how does it affect the Congress races, and what happens afterwards. The POD can take place at any point in time, doesn't have to be 2015 or 2016.
FYI, this will most likely get removed unless you move it to chat. Morever, this forum's base is split overwhelmingly between the center and the far-left. There's barely anyone here who will give you an unbiased answer. You'd have better luck somewhere else.
I think, with his domestic policy agenda being blocked by the Republican Congress (or at least the Republican House), Sanders' term will be defined by its foreign policy. As we haven't seen what the next four years hold for us in terms of international events, it's hard to say how his administration would turn out.