The chemical definition for litmus disambiguates to the political expression "litmus test."
The OED says that the political usage first appeared in 1957. link
WTH happened in 1957?
[Update: Twitter friend Meadabawdy reports that Merriam-Webster dates the usage of "Litmus Test" back to 1952: link]
That puts the coinage squarely in the McCarthy era.
[Originally posted here with plenty of "interesting" comments]
