Before the civil war: the United States “are”.
After the civil war: the United States “is”.
The original US Constitutional republic died the day the civil war ended, and a bureaucratic federal oligarchy took its place.
More accurately:
Before the Constitution: the united states “are”.
After the Constitution: the United States “is”.
Yet on the "repeal the 17th amendment" threads, the same crowd argued this occurred when the 17th amendment was passed in 1913. Your arguments might be more convincing if you could pick one date when "our Republic died" and stick with it, instead of setting it decades apart depending on your talking point on that given day.