The attack ads practically write themselves. All they had to do was point out that Chavez, in Venezuela, was an actual Socialist, and look how well Venezuela is doing, and Bernie will do for America what Chavez did for Venezuela. Repeat at high volume for 3 months. Heck, look at how well the attacks claiming Obama and Bill Clinton were Socialists worked, and they were nowhere near socialism.
Bernie, unlike Hillary, would've lost the popular vote too.