In politics or political discussion, a purity test is a tactic intended to question how ideologically committed a group or individual is to principles they claim to stand for or which others claim they stand for. It may be employed for various purposes, including to influence the target, to signal ideological position or distinction, or to discredit the target as being impure, inconsistent, or hypocritical.
Sherwood, Jeff. "On the radar: purity test."Oxford Dictionaries Blog. March 1, 2017. http://blog.oxforddictionaries.com/2017/03/on-the-radar-purity-test/.