As explained well here. In small part:
. . . Trump is unquestionably unpresidential (this is the man who mocked a disabled reporter), but he’s nothing like Hitler. I encourage everyone to challenge and condemn the immoral and unfeasible ideas that Trump has, such as the proposal to deport 11 million undocumented migrants, but comparing Trump to Hitler is not only inaccurate, ineffective, dishonest, and dangerous, it also trivializes the tragedy of the Holocaust in the name of scoring political points. . . .
But to compare the nature of Trump’s appeal and his rhetoric to Hitler’s . . . and to note, as I have, repeatedly, that, according to his ex-wife, he frequently read from a book of Hitler’s speeches by his bedside . . . and to note his affinity for journalist-murdering dictators like Putin and Duterte . . . and to find it telling that he defended some “very fine people” among the white nationalists carrying torches in Charlottesville chanting “Jews will not replace us” — well, it seems to me, these are data points that someone evaluating Trump’s world view and fitness to be president should take into account.