There's a widespread misconception in modern life that you can't, in polite debate, call your opponent a Nazi. Totally untrue. It's a dangerous game, but with the right tools, you can play it. So without further ado, here are the four easy steps to implying that a perfectly rational human being is, in fact, a jackbooted stormtrooper thug.
1. Don't use the words "Nazi," "Hitler," or "Holocaust." Too strong, too obvious. Refer to these things obliquely: by description, or by year. This way, you sound more learned and intellectually rigorous. Oh no, you're definitely not accusing anyone of mass murder here. There's just an apt comparison you would be remiss not to make.
2. Don't insult. Illustrate the similarities. Directly relating someone to Nazism will only get everyone up in arms. Instead, you should be drawing historical analogues. Making parallels clear. But no more than that. This is potent stuff, like fissile material. Too much of it in one place, and you just might get a runaway reaction. It's enough to put someone in the same room as it, and let the audience do the rest of the work. If you make the final connection yourself, you've advanced a bridge too far.
3. Never close with the Third Reich. Everyone knows the Nazi comparison is the strongest rhetorical weapon in the book, and so a lot of people want to build to it, grinding slowly to the finish, and then unleashing it in a final mighty blast. Amateur mistake: it only calls attention to your (alleged) rhetorical excess. Follow up it up with another, less explosive analogy, and you'll look methodical, dogged, and exacting. You see, you said it because it needed to be said; you'll keep making these and other comparisons until the other side sees the fundamental soundness of your position.
4. Never apologize. You did what you had to do. History will prove you right. Sometimes great misbehavior has to be met with great force. Explaining yourself, preemptively or otherwise, just makes you look weak. Have confidence in what you did and everyone will assume it was the right choice afterward.
If you follow these steps, you can totally get away with calling someone a Nazi. Everyone listening should come away quite convinced of your reasonableness, but with the vague and not-quite-definable impression that your opponents are, in fact, horrible, unforgivable war criminals.