When someone on the left wants to denigrate someone on the right in the most extreme way, they call us a Nazi or fascist (I 100% reject the notion that Nazism or fascism is of the right, but that is for another time). When someone on the right wants to denigrate the left in the most extreme way, we call them a communist. The difference here is that while leading figures of the Republican Party, or the right broadly speaking, roundly reject and condemn fascism, many on the left still find communism acceptable.
There is a fringe that has attached itself to the right, known as the alt-right, that preaches white nationalism, but that small group of basement dwellers has been routinely and vocally repudiated by leading figures of the actual right. The same cannot be said for the left. Examples abound.