Bernie has 2 base problems:
1. Gun Control Nazis who don’t like his relatively benign stance on the Second Amendment.
2. Paid Clinton Machine hecklers.
The Democratic Party establishment has its own base problems. Up until 2008, Democrats like Bill Clinton and Al Gore at least paid some lip service to supporting the interests of working and lower middle class whites in small towns and rural areas. Obama wrote this constituency off completely and won by appealing to the trendy youth, to urban yuppie elites, and “minorities.” Obama could win because of record minority turnout at the polls. I doubt that Hillary can get out the vote in the inner cities and the barrios like Obama could, and at this point working and lower middle class white voters (even including union workers) have mostly written off the Democrats and won’t be coming back.