And let's face it, you're not likely to see a one-for-one repeat of what Big Daddy and the Sunshine Friends did. The situation that sparked it- namely the Versailles Treaty and the limitations on the German army- meant that the SA could grow large enough to overwhelm the legitimate authorities. And that's not likely to happen in the US (Iraq and Syria, on the other hand...). But that doesn't mean that there aren't important similarities that will, if left unchecked, lead to a 3rd Reich style situation.
1) Round up all the immigrants and deport them, huh?
Y'know, once mass expulsion was on the table at all, my ears sure started to twitch. It doesn't take much for expulsions to change into worse shit. Even if Ike tried it, it was wrong then and it's still wrong now. Even FDR fucked up sometimes.
Oh yeah, speaking of which, let's look at what immigration looks like currently, shall we?
Furthermore, there's definitely no way that this kind of policy could turn into persecution of brown people for looking brown, or causing problems for genuine asylum seekers. Nope. Not at alllll.
2) Register all the Muslims in a database, huh?Can't think of any way that could go Nazi in a hurry.
3) The War Crimes. Oh man, the War Crimes.
Now, Trump isn't the only one promising to carry out War Crimes. But between the Sippenhaft thing, the illegal torture thing, and agreeing with Cruz on the broadbase bombing thing, there's a lot here to make the Geneva Convention curl back in a corner and beg daddy to stop drinking.
4) Limitations on Freedom of the Press.
No, you can't actually re-write libel laws or blow up the existing precedent on libel laws so that people can't say mean things about you. But thanks for playing.
5) Popular anger that blames economic stagnation on ethnic minorities cast as disloyal.
This is the big one, and the one that is like, whether or not Trump himself is a good Hitler cognate, his supporters are a REALLY good NSDAP, Polish populace cognate. The anger and fear and hatred towards non-whites and immigrants is really very... 1933 Germany and Poland. The problem here isn't exactly Trump himself. But the presence of Trump gives voice to a lot of these elements, and that is the Hitlerian part. Also, if it wasn't Trump riling people up, it'd likely be Cruz, who is just mildly better at not peppering his rhetoric with promises of war crimes.
But until the people who think that voting for Trump is a good idea because either a) they agree with all the jingoistic, nationalistic bullshit he spews or because b) they don't really care about the jingoistic, nationalistic bullshit part, but think somehow he's going to make good paying jobs appear, get past their very narrow, personal, All For Me And Mine perspectives, we are going to keep having these demagogues crop up dripping rhetoric that wouldn't be out of place for the opening number of Darling Adolphe's Dancing Stormtrooper Review.