In their minds: Anyone that supports Stalin.

Roughly, the argument is that the nationalistic and culturally conservative elements of the USSR, particularly under Stalin, combined with the relationship between the heads of the party and those in charge of the industries, made it close enough to fascism to warrant the label.

I’m not fond of it because, for all the USSR’s faults its ideological agenda was never in line with fascism’s so the comparison is not helpful in understanding the Soviet system.