The US Left doesn’t quite exist.
Everything you might call “the socialist movement” — anarchists, Marxists, democratic socialists — is on the edges of a bigger phenomenon. A few months ago a multi-year study was published, finding that the strongest predictor of an individual’s politics isn’t demographics, party affiliation, or even self-identified ideology. Rather, the US has seven distinct segments with their own ideologies, demographics, social networks, and “basic values” (for instance, how they raise children). US politics happens through that set of political subcultures.