Trump is an existential threat to democracy here and throughout the world.
Why do you think this? What do you think Trump will do that will destroy democracy throughout the whole world?
The whole world might be a stretch, but it's not what he will do, rather what he won't do.
Russia will invade (the rest of) Ukraine, and perhaps some other European countries. Trump won't try to stop this, nor even help those countries to resist.
China will invade Taiwan, and perhaps some other South East Asian countries. Trump won't try to stop this, nor even help those countries to resist.
North Korea will try to invade South Korea. Trump will support this attempt (albeit with words rather than deeds), even though it will probably fail, and will cause millions of civilian deaths in the process, because he and Kim are besties.
An isolationist America is bad for the world; An America whose President actively admires foreign dictators, and who dislikes other democratic nations (whose populations are rarely brought to book for saying
mean things about Donald Trump) is disastrous.