There really isn't an equivalent. For the most part the fringe left doesn't like the Democratic Party enough to even attempt to contest the primary and wouldn't be remotely viable even if they tried to contest it.
There is no equivalent to Trump. Trump is a massively successful businessman whose views are often informed by his elite experiences and what would benefit him, which is why he's a Republican and why so many other wealthy businessmen are Republicans. In contrast, most of the extreme far-left viewpoints are the same thing for poor losers, e.g. all drugs should be legal, the government should pay me to do whatever I want all day, and tax all income over $100,000 or just throw the executives in jail and ban business. There are no successful or wealthy people who have these views, it's mainly loony activists, narcobums and pothead internet addicts. Unfortunately when you're not successful and wealthy you tend to also not be tremendously famous or have thirty years of people admiring you and begging you to run for president. With someone like Kanye West I think you'd find that his actual views on things aren't particularly loony or ridiculous, as rich and successful left-wing people tend to not have these extreme kinds of views (there are absolute idiots, to be sure, like Susan Sarandon, but with them it's more about an extremist worldview than actual extreme policies), the problem is just that he's so poorly informed and likely believes in a few conspiracy theories (e.g. "I treat the cash the way the government treats AIDS, I won’t be satisfied til all my niggas get it, get it?") but he's not gonna advocate for a policy of banning all white people from the country or anything.
