> I think you're being downvoting because your first sentence appears to be calling all leftists lunatics when (correct me if I'm wrong) you're actually saying "there are no leftists who are quite as loony and prominent as Trump so there is no easy comparison".

I think so. The GP's use of the term "leftist lunatics" reads as a right-wing shibboleth to me.

Outside the US, I hear "looney left" and related terms used all the time by political moderates to describe any leftist groups outside the political mainstream, with a similar connotation to "alt-right", though it's a meaner phrase. It's derogatory towards them, but not towards the left in general.

Inside the US, "looney left", "communists", and "traitors" are all used by the political center-right and right-wards to describe any leftist group. Any reference to the left is intended to be derogatory.