Originally a scientific term, the word has been used politically for at least 80 years
When it spiked
February 4, 2016
Lookups for litmus test spiked after Democratic presidential candidates Hillary Clinton and Bernie Sanders confronted each other at a town hall forum in New Hampshire last night. Asked whether she would have a litmus test for potential Supreme Court justices, Clinton replied: "I have a bunch of litmus tests...I’m looking for people who understand how the real world works, who don’t have a knee-jerk reaction to support business, to support the idea that money is speech.”
The political meaning of litmus test as "a test in which a single indicator, such as an attitude, event, or fact, is decisive" is so common that it's the term's definition in our dictionary. But litmus test didn’t have very much to do with politics for the first century or so that it was in use. It began as a chemistry term, and referred to the process by which a substance was determined to be alkaline or acidic. This was done by dipping a strip of litmus test paper into a solution of the thing that was being tested; the resultant change in color of the paper would inform the experimenter whether they had an alkaline or an acid. The earliest known uses of litmus test , going back to 1824, are all with reference to paper.
Litmus test papers are prepared by staining slips of paper, four inches long, and half an inch wide, with the tincture. (1824, John Griffin, Chemical Reactions: a Series of Amusing and Instructive Experiments)
By the middle of the 19th century we begin to find litmus test used on its own, without reference to paper slips, although at this point it appears to still only be found in scientific settings, or in reference to the brewing of beer, which can be quite scientific.
Their fermentations, however, generally speaking, are carried on at as high temperatures, or nearly so, as those used by the vinegar-makers; but when successfully conducted, their fully attenuated wash will bear the litmus test, as well as sound beer. (1844, William Black, A Practical Treatise on Brewing)
In the early 20th century litmus test began to be used in a figurative fashion, with the sense that it most often has today.
It became more and more apparent that this criticism was valuable only as a sort of litmus test indicator of American culture. (1927, Edwin Harold Eby, (dissertation) American Romantic Criticism, 1815 to 1850)
Under the Colucci process the bivalves are subjected to a sort of litmus test, performed with a large clove of garlic. (1935, August 20, San Francisco Chronicle)
It takes time to fit the pieces together in an inquiry of this kind. The hasty litmus test has no place in it. A grand jury inquiry is not a trial jury. (1950, August 5, New York Times)
We have been using litmus test in political jargon for at least 80 years now —an article from the Washington D.C. Evening Star from 1936 contains the line “even if Maine isn’t what it used to be as a political litmus test”). It appears to be seeing increased use in this context in recent decades, particularly with regards to a political candidate’s intentions for appointing judicial appointees.