Obama was compared to Stalin
more than once despite being a fairly moderate politician. So even though the initial claim was not predicated on anyone calling a left politician "literally Stalin" there are in fact many instance of right wingers comparing left wing politicians to Stalin and Hitler. But I'm sure you'll move the goalposts in such a wya that this doesn't count.
Calling him "
Literally Hitler" is usually done facetiously since he's obviously can't be
literally Hitler. Do people compare Trump to Hitler and other fascists though? Sure but left wing politicians also get compared to radical elements on the left like communists and authoritarian socialists. And of course Obama was himself compared to Hitler countless times before Trump ever was.