Any concept of "moving the Democratic Party to the left", however one phrases it is, I'm afraid, a fool's errand. The Dems are part of the apparatus of elite rule. As such, demanding they make a "choice" makes as much sense as demanding that ExxonMobil must decide whether to make money while destroying the environment or become a non-profit. It makes as much sense as a bunch of chickens running into a den of foxes to persuade them to be vegetarians! There is no choice to make, and it's not about ideas. Politics is about the clash of interests, and power. Not ideas.
We must work to make sure Bernie gets every single vote possible. And in the event he is cheated out of the nomination which seems likely, we need a Plan B. More on that at a later time.
P.S. Anybody with doubts about this should go to the National Endowment for Democracy website and scroll down the list of directors. This is a ruling class institution, implementing policies around the world as determined by the interests of American business and the ideology of the neocons. Dems, Repubs and AFLCIO traitors are equally represented, and they get along with no friction whatever. In fact, from the "About" page, they brag about it: "From its beginning, NED has remained steadfastly bipartisan. Created jointly by Republicans and Democrats, NED is governed by a board balanced between both parties and enjoys Congressional support across the political spectrum." Nuff said?