You took it a bridge too far - I don't recall many people actually calling Mitt Romney anything like a Nazi. He was accused of being a wealthy elitist and vicious corporate capitalist who openly despised the poor and was so out of touch with humanity that he had his emotions simulated by an algorithm. But not "Literally Hitler."
Your memory is selective. Romney was called a Nazi (or compared to one) by actual Democrat Party officials. Ryan was compared to Goebbels. Nikki Haley was compared to Eva Braun. Romney wasted a bunch of time demanding apologies, etc.
It doesn't matter -- as I said, no one pays attention to it anymore, really, and Trump, at least, understands that.
Trump is more of a William Jennings Bryan or (at worst) an Andrew Jackson. And likewise, it'd be best if Hillary proved not to be an Evita Peron.
Given that, as far as I know, none of the above ever launched a global war of domination, nor murdered millions of people for belonging to a specific ethnicity, I'm pretty sure that none of those guys were Hitler.
The problem is that the opposition has cried Hitler so many times that no one (other than their fellow rabid partisans) pays the slightest bit of attention any more. Certainly I don't. Nor does Donald Trump. A nice guy like Mitt Romney gets upset when people call him a Nazi and tries to prove that he isn't one. Trump doesn't give a shit. He's the honey badger of candidates.
That's going to suck pretty bad if an actual Hitler ever comes along. I think there's some kind of old fable about that, isn't there?
Note: I do not like Donald Trump and am not planning on voting for him. I expect him to win handily, though, so we'd better hope he's not, you know, actually Hitler.