One case is irrelevant to another on an important formal level. The difference is that Obama, Bush and Trump are distinct persons. That means that the charge can be (and indeed must be) evaluated independently in each case. I would say it is false in each case but that does not justify a conclusion that all such accusations are equivalent. They may all be false for different reasons, or false in different degrees. Each case should be tried on its merits even if you exonerate everybody. Who knows, the charge might be true--a lot of people have said Trump is another Hitler, or sorta like Hitler, or shows (the dog whistle version) fascist tendencies that remind us of a former age.
If you have two guys charged with robbery, it may be that neither of them is guilty, but you do not lump the cases together for purposes of finding out.
In the case of Trump, a couple things stand out, the first being the idea of many of his supporters that they have altogether enough government going on around them, indeed too much in some areas of their lives. They are not looking for a dictator to straighten out Germany and assure the volk's right place in the world. Instead, they long to be let alone. So on a basic philosophical level there is a big disconnect with Hitlerian aims.
Another difference is that while Obama and Bush were compared to Hitler now and then, it did not have much audibility, and hardly anyone took such talk seriously, but there is in the Trump-Hitler comparison (or was, it seems to be dying down a bit) a loud drumbeat in the press, in entertainment circles, in academia--lots of people talking that way, even Noam Chomsky, whom one would expect to have better sense. Even Meryl Streep, who charmed us so in The Deer Hunter and other memorable roles. Even reporters who took a break from their five W's to indulge in a bit of Trump bashing.
When I raised my question here, I was hoping for something more along the lines of a reasoned examination of the matter, rather like this piece that ran in Der Speigel:
http://www.spiegel.de/international/zei ... 22035.htmlThat article does not exonerate Trump of the charge, but it does raise reasonable doubt. I do not think it goes far enough to uncover evidence tending to clear him, but it at least engages the question of truth or falsehood of the charge. What I got when I came here was very different--irrelevancies leavened with persiflage, mainly.