Prior to his election as Chancellor, Hitler had already led a failed terrorist uprising in Munich, spent a long time in prison subsequently, written an extremely racially charged ideologically aggressive book, expounded extreme anti semitic views, gathered up his own mob to put pressure on his opponents, recruited many known anti semitic into his ranks etc.
That's why the Hitler comparisons are unhelpful and utterly daft. Hitler was one of the worst to ever live. DT is gonna be crap and make some people's lives less comfortable than need be the case, but the Hitler remarks are hysteria driven nonsense.
They are daft, but there are some common threads between the rise of individuals / movements that are extreme and the rise of Trump.
For a start, domestic politics has to be in a complete and utter (and usually corrupt) mess - look at what was going on in Weimar Germany between 1930 and late 1932 for instance, or what was happening in Iraq before Maliki got booted out, or Russia between 1905 and 1917. A state has to be diseased before people turn to extreme elements. Secondly, enough people have to be suffering - Germany had seen the generation before cut down in an especially needless war and had gone through four or five years of economic calamity; Iraq was the most corrupt country on earth, Russia was run by first inept autocrats and then, after February 1917, bickering nonentities who oversaw calamities and humiliation in war, starvation in the countryside and savage oppression of even well-intentioned criticism and reform.
Finally the rulers have to utterly mishandle the extremist movement for it to succeed. The Weimar regime let the Nazis get away with murder (literally); Maliki went after the pro-US Sunni Awakening groups who were the only possible alternative to what became IS, and the post-February Russian provisional government both allowed the Bolsheviks to operate and failed to remove the glaring problem (the war) that caused the Tsarist regime to collapse.
Of course, the US is not in as bad a place as those states were. However, their domestic politics is demonstrably in a mess, people are suffering on a considerable level and across communities / racial lines, and there is every likelyhood that the defeated of left and right will select the easy option of mouthing off, calling for "resistance" to Trump and further dividing the nation, rather than taking a good long look at themselves and trying to fix things.