Democrats have been suffering huge losses ever since Obama pushed them to the left
Democrats have been suffering landslide defeats ever since Obama forced his despised "Unaffordable Care Act" into law. For them to think that they can improve their position by moving further to the left looks delusional at this point. It is reminiscent of the Islamist who think things are going bad for them in the Middle East because they are not Islamic enough. Then you get al Qaeda and ISIL competing to see who can be the most extreme Islamist.
After losing the presidency, Congress and lower-level races, the party is divided over how aggressively to position itself on the economic left.