The problem with that is that some people seem to be under the mistaken belief that the many hardcore Sandernistas would vote for Hillary in the general election. They are dead wrong. Trust me, any average Sandernista has a hatred for Hillary Clinton, and for good reason. Not only that, but the Sanders campaign has got many people involved in politics, and r/sandersforpresident mobilizes its members amazingly. Honestly, if Sanders lost the nomination, r/sandersforpresident would likely become r/steinforpresident. After that, it'd be the 2000 election all over again, and everything would go to hell for the Democratic Party.
The denizens of r/Sanderforpresident, thankfully, are not representative of the average Sanders supporter. The polling shows that most Sanders supporters have a positive or neutral opinion of Clinton.
