Litmus test (politics)
In politics, a litmus test is a question asked of a potential candidate for high office, the answer to which would determine whether the nominating official would proceed with the appointment or nomination. The expression is a metaphor based on the litmus test in chemistry, in which one is able to test the general acidity of a substance, but not its exact pH. Those who must approve a nominee may also be said to apply a litmus test to determine whether the nominee will receive their vote.