What is an existential threat?

I can't figure out why the left thinks Trump was/is an "existential" threat. What does that even mean? That he might run out on our allies and Americans in the middle east? That he might increase our taxes? That he might hire enough IRS agents to populate a small city? That he might drive the economy into inflation and recession at the same time? That he might lead the charge against freedom of speech? That he might collude with the press? That he might encourage the FBI to attack his political enemies? That he might worsen the relationship with China? That he might destroy the fossil fuel industry condemning us to a return to the pre-industrial age? That he might encourage WWIII?

Who exactly is a threat to the United States? I think he's living in the White House, not in Trump Towers.