I don't think the Democrat establishment is so retarded as to deny Bernie the nomination if he wins the popular primary. In the short term it would splinter the party and be a boon to Republicans, if for no other reason than decreased voter turnout. In the medium-term it would just pour gasoline on the anti-establishment fire, and there would be hell to pay in 2020.
I could definitely see it tank the voter turn out for the Democrats if that happened. They would essentially be handing over the White House to the GOP.