After a referendum , the Constitution was amended in 1959 , guaranteeing Government protection of the civilian population in times of national catastrophe.
When I was there in the early eighties, there was a bunker close by , wherever you went, four to five million of them, house building code was air tight cellar doors, every citizen had three weeks of gov issue survival rations etc.
Of course the threat was from the Ivans, with presumably x minutes of warning of incoming, and x steps to the shelters.
I don't think you could "fully" protect the population from the asshats with a nuke in say, Philidelphia. But devastating as that might be, it's a pinprick with reference to the survivabilty of the United States as a political entity.
"fully" protected isn't doable, but we (humans ) are terrible judges of risk.
We scare easy over bee stings.