Both main US parties aren't really "left", in a UK sense at least, so it's no surprise. The fallout between the actual Left and the Democrats probably came to a head when Bernie Sanders was running for candidacy, and he really *is* a leftist candidate -- not a Commie and not as left-wing as Corbyn but about as close as you can get in the US. But anyway, he lost to Hillary and I don't think that his supporters, at least not his most passionate ones, will ever quite forgive the Democratic Party for that.