*litmus test*

1. (Chem.) A test to determine acidity or alkalinity, using litmus as an inddicator. [ PJC ]

2. Hence: (Fig.) A test for a single factor, which has only two outcomes, positive or negative; specifically: (Politics) the question of whether a candidate for office is for or against a particular position on an issue; -- used, e.g. by voters concerned predominantly with a single issue, to decide whether to vote for or against a candidate; as, many voters take a candidate's position on abortion as their litmus test. [ PJC ]