Dims blew that when they nominated Clinton over Bernie.
If Bernie were the nominee, he'd get crushed like Mondale did. There's a reason Republicans didn't attack him in the primary: they wanted to run against a crazy old socialist. They would have flooded the airwaves with ads like this and this and this and he'd lose probably the same map that George McGovern did in 1972.
Translation: Even though Millenials are much more left wing than previous generations, and this was well-noted by Frank Lutz, you are wanting to believe that a nominee Bernie Sanders would lose so badly that the likes of California, New York, Illinois, Pennsylvania, Michigan, New Jersey, Washington, Massachusetts, Maryland, Wisconsin, and many states which have not carried Republican since the 1990s would vote for the same candidate as you: Donald Trump.