Well, anyone right of Richard Spencer these days is typically called, "Communist", "liberal", "globalist", "leftist", "BLM terrorist", "Antifa", and "far left extremist" interchangeably by the side that's been working very hard to make sure words don't mean anything anymore.

But to leftists, the distinction is still important: leftists believe in Marx's idea of a class struggle. Most other Democrats, on the other hand, don't even know what that is.

The class struggle goes like this: what's good for the miner will never be good for the mine owner. What's good for the line cook will never be good for the restaurant owner. What's good for the actor will never be good for the studio executive. And so on and so forth.

The reason these two sides are inherently at odds is because every penny paid to workers is a penny NOT made in profit. And likewise every penny made in profit is a penny NOT paid to workers. If workers score by stealing points from bosses, and bosses score by stealing points from workers, then workers and bosses are on different teams.

Bernie makes allusions to this notion constantly by heavily using the phrase "working class". Plus his proposals are pretty anti-capitalist (cancelling student loan debt, Medicare for all). So leftists flocked to his banner, elated.