Anecdotically, in the Southern American country I'm living in, a country now filled with immigrating Russian families, almost all these new (and legal) Russian migrants, if you ask them, think that the situation in Europe will end up with a nuclear war. They're convinced that Biden and Zelensky will somehow succeed in starting a nuclear event. First by sending tanks, then warplanes, then long range missiles, and so on...
If they're correct, then in the event of a nuclear strike, even if it were limited solely to Ukraine, I think it would cause mass panic here in the US. Once people realize that the threshold of an enemy using nukes against American interests has been breached, the threat would become real in people's minds. The last couple of years with COVID has provided a glimpse of what we'd be likely to see next: grocery store shelves emptying out, people fleeing the cities, massive social unrest, etc. Nothing has to be detonated here or within the West for it to cause massive turmoil in our society.