"Dear Americans," the post on my Facebook wall began. "Go ahead, vote for the guy with the loud voice who hates minorities, threatens to imprison his opponents, doesn't give a f*** about democracy, and claims he alone can fix everything. What could possibly go wrong…? Good luck."

Signed: "The people of Germany."

As an American Jew living in Berlin since 1997, I have a special fondness for my deeply principled German friends. Usually, they avoid comparing modern politicians to Adolf Hitler. And it's right they do. It's especially unseemly when Germans relativise the Shoah.

But this ironic Facebook post rings true. The thought of a Donald Trump presidency makes me sick with worry.

I know - as an observer of politics, culture and society in Germany - that the far-right is waiting to catch a ride on a populist wave. Brexit should have been a warning.

Mr Trump has tapped into legitimate fears and long-time resentments of some Americans and, yes, their desire for a better life. At the same time, he is sending a message to the likes of Timothy McVeigh and David Duke that it's OK to come out now.

Strangely, this does not surprise me. Especially since the US Supreme Court had to resolve our disputed 2000 presidential election, I've felt a deepening divide in the USA.

But what does surprise me is that some friends and family, including those who say "never again", supported a man who speaks of jailing his opponents, of not accepting the election results unless he wins, of punishing women who have abortions, of planning mass deportations.

A man who admires Vladimir Putin and other demagogues. Who spent years claiming that our first African American president, Barack Obama, was not born in America, and who recently spent mere seconds reversing that claim, with no apology. A man whose slogan, "Make America Great Again", implies that this country - where people of all backgrounds can succeed, as our own ancestors did - is not great any more.

Did Trump really mean it when he told the Republican Jewish Coalition last year, "You're not gonna support me because I don't want your money. You want to control your politicians, that's fine"? Is he clever for talking about so-called international banking conspiracies and for highlighting prominent Jews in a campaign advert about the greatest threats to society? Does it matter that hate groups are rushing to back him?

"Trump's antisemitic supporters do not scare me in the least," said one Jewish friend, who called the Republican candidate "an inexperienced politician and an unpleasant, provocative, and even a vile personality". But "Hillary's antisemitic, anti-Zionist supporters are mainstream in her party and scare me greatly," the friend wrote. And a relative wrote: "Trump has severe character defects but may surprise you by becoming a great president." So it seems a few friends and relatives held their noses and voted for Mr Trump - as a recent poll says 19 per cent of Jews probably would.

I was brought up in a politically liberal home, motivated by essential Jewish values. To cite Hillel: "That which is hateful to you, do not do to your fellow." And, "If I am not for myself, who will be for me? But if I am only for myself, what am I?"

So I felt shaken when I heard the arguments of Jews for Mr Trump, the same Jews who would say their brethren should have recognised the dangers of Nazi Germany and fled.

I see this conflict between Americans of all stripes as more ominous than the so-called clash of civilisations. We are divided, and thus more easily conquered. There are surely some who are toasting our instability - perhaps with vodka.

No, friends, this is not exactly 1930s Germany all over again. This is uncharted territory, and global stability is at stake.