Consider saying “the United States are” in cases when you believe it is appropriate.
Abraham Lincoln publicized referring to the nation singularly during the Civil War, and this revised phrasing was soon widely adopted. Abe’s goal was to unify the nation, and our modern grammar reflects that.
I’m not sure I agree. The United States is named as such because originally the Articles of Confederation created loosely bound group of independent countries (thus they would be the “United States” because a is basically an independent country). So if things had stayed like that, referring to the United States in the plural and the individual countries in the singular would make sense. I.e. The United States are a group of countries. South Carolina is one of them.
But since we dropped the Articles and drafted a Constitution, referring to the U.S.A. in the plural is less sensible. The United States is just one country so using the plural verb doesn’t work so well. You wouldn’t say “Italy are shaped like a boot”, because even though Italy is made up of different local regions, it’s still one country. The United States is just one country that happens to have an antiquated name.
Admittedly, if you wanted to refer to “the states”, using are would be appropriate. I.e. The states are holding primaries.