Is leading such mass mobilization something that the President can do well? Can the President go beyond using the "bully pulpit" to advocate for those causes. Sanders has called President Obama a "disappointment," but Obama has not been remiss in making speeches in favor of reforms.
Over the last eight years there has been a significant upsurge in militant social movements. Occupy and Black Lives Matter have put many people in the streets and placed new demands on the table. There has been a steady growth in email/social media based mass mobilizations across a wide range of issues and groups. The social movements are here. Would they be stronger if President Obama had explicitly endorsed them? In all likelihood, that kind of endorsement would have not helped and probably been repelled. Remember the deep suspicion that MoveOn aroused in the Occupy Movement? Occupy was very frightened of being co-opted by the Democratic Party.
The independent social movements are doing fine. Strengthening them is the work of the organizers at the grass roots level, not the President of the United States. No President, not even Bernie Sanders, is going to put a lot of effort into building a social movement that is critical of the President. Nobody is going to join a social movement that is in the tank for the President.
The second problem with the Sanders theory of change is that eventually all efforts at reform come down to legislation and legislation takes compromise and compromise takes flexibility. And legislative negotiation occurs both in public at a symbolic level and at the level of actually writing legislation and spending money. And these levels are intertwined.
One of the things that Obama learned in negotiating with the GOP was that it was a mistake to want something too publicly, or to refuse something too publicly. As soon as something went public, it became both a target of the opposition and a non-negotiable to the public on his side. This is what happened to the public option in the ACA. Once it became clear that it symbolically important to progressive Democrats, it became completely unacceptable to the GOP. And then it became something that 'center' Democrats like Lieberman and Ben Nelson had to oppose because doing so would prove their independence.
Remember the proposal to let 55 year olds buy into Medicare? A perfectly reasonable proposal that would have taken older sicker people out of the insurance pool. Joe Lieberman shot it down because it made the liberals too happy. Who knows, it might have passed if hadn't become a public symbol.
Mass movements cannot fruitfully intervene in legislative negotiation. They will be demoralized by the process. Just as the progressive movement got demoralized by the failure to get the public option included in the ACA.
To be truthful, I don't think Bernie Sanders wants to be President. I think Bernie Sanders wants to lead a political revolution against economic inequality and the corruption of our campaign financing system. For that I wish him every success. And I think that becoming President would be a disaster for his true calling.