True that lumping the old right's sins with those of the Nazis threatens to erase their sins when they can deny the kinship. I had this argument long ago with my stepson. As a child he absorbed some pretty leftist ideas (whoda thunkit?) but as adolescents sometimes do, he oversimplified a bit, and tended to portray his chosen foes as literal Nazis and storm troopers. While appreciating the sentiment I had to point out that if they can fairly easily disavow the hyperbole, they sort of win the argument. You have to nail their sins for what they are, not what they look like.
But that said, I can sympathize with NM156, when it's become the standard form of discourse, and the opprobrious rightwingers call everyone who opposes turning the armed forces into domestic secret police and masked gunmen leading former DACA children into cages with chains, etc. a traitor and a communist. As usual, when engaging with the violent and the insane, it is very hard for the sane to win without becoming as bad as they are.