In the 14th Century, Spanish chemist Arnaldus de Villa Nova discovered that litmus, a compound derived from natural bacteria, would turn red in acidic substances, and blue in alkaline.

In American politics, the ‘litmus test’ is most particularly applied to incoming justices of the Supreme Court. Its genius lies in its simplicity: why bother with a complex suite of policy assessments, when a simple, binary proposition will do?

There is no question that in the approach to the midterms, America’s body politic is acidic.