Maybe Sanders does better than Obama with Hispanics. Maybe. But we're already reaching a ceiling there. He will obviously do much worse with blacks, and Trump is just the kind of Republican that could really make inroads with them (maybe winning up to 15%, which makes a difference in swing states). And then Sanders will get clobbered with moderates and suburbanites. You think ordinary middle class swing voters in Northern Virginia or the Philly burbs are gonna vote for a 75-year-old socialist?? Trump is going to go hard to the middle once he's won the primaries, he'll de-emphasize the crazy stuff and triangulate, and he'll win the general.
This. Sanders is a joke, yet he has a whole lot of people convinced that he can win over everyone. It's unrealistic to think that he can win the necessary votes.
