With respect, on the reasons you’ve set out many other countries would have just as good if not a better claim to being the greatest and most moral country in the world.
The U.K., for example, banned slavery long before the U.S. and it was rather more effective at beginning to eliminate it entirely because the Empire was still going strong. Similarly Britain has fought in almost every major conflict and peacekeeping mission of the 20th Century, we were in WWII years before the U.S. started to provide real help, not to mention coming close to bankrupting the country funding the war which eventually freed Europe and North Africa from Hitler. More recently it was Britain who wanted to finish off the Gulf War properly first time round, overruled by America and just look how thats turned out. As for welfare sixty years on you still don’t have a jewel in the crown comparable to the NHS, Britain was one of the first to start building a Welfare State which provided from “cradle to grave”. America has also been the Superpower for a comparatively short time compared to those who would have held the title in the past.
Obviously we each have our own bias on patriotism, but turning a thread into a love in for how great America is seems a bit much.
Back on topic. The main concern seems to be a repeat scenario of a party like the Nazis who dress up their real agenda behind enough good policies to form a winning majority. I think for the U.S. it would be just about impossible for such a think to happen, for it to occur the President would have to be able to exert massive control over all branches of government, he’d need a huge majority in both Houses, the support of more than half of the Supreme Court, at least the majority of States and more terms in power than he could currently hold not to mention the backing of the Armed Forces. Part of the problem which allowed Hitler to get in was Hindenberg needing his support, this allowed the Nazis to get into power and claim ‘support’ from the establishment figure of politics at the time. It also came in the wake of a massive defeat, and as we all know America has never lost a war

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If any President did try to get by such policies through executive order they would be repealed or ruled unconstitutional in fairly short order. The nature of a two term Presidency also means he would seldom if ever nominate a majority of the Supreme Court, and even then under scrutiny before Congress they probably wouldn’t be approved once their politics came to light.