How would you have sounded the warning? What language would you have used, what referents? What kind of skulduggery, exactly, were Hitler's miscreants up to, in the eyes of his contemporaries?

We have the benefit of spoilers, today. We know how that story turned out, and warning against it, today, is as simple as saying "like the Nazis!", which everyone hearing expands into a rich knowledge base covering the Third Reich and What Happened. You're using a similar shorthand, by citing sections of written volumes to make your case.

History can never repeat itself, only rhyme, because the knowledge of history interferes with attempts to repeat it, another analog of Heisenberg.

Trump isn't smart enough even to read Shirer as a literal handbook, let alone think up the necessary adaptations that take advantage of circumstances in today's world, like the Internet. But he is the hapless beneficiary of people who are smart enough to adapt Hitler's anti-democracy playbook to the modern world. Putin understood how to weaponize social media, and helped elect Trump. Rightwing gunslingers like Paul Manafort weaponized PR into a tool for taking over whole nations like the Ukraine. Trump may be instinctively attacking our democratic institutions, like a cornered rat; but he's being enabled by people who do understand what they're attacking, and do have an, unfortunately, well-thought-out plan to destroy democracy in America.

Trump owes a lot to the Koch brothers and their Network, to name another benefactor, this one ruefully inadvertent.

Everything hinges on the ferocity of the backlash in the 2018 mid-terms. Fighting back is a more realistic option in 2018 than it was in 1933. Remember, that epochal vote might have gone against the Nazis; there was one moment when the future hung in the balance, and the hinge was the literal last act of a democracy.

Just one election. That's all it takes.