[1] During United States presidential election campaigns, litmus tests the nominees might use are more fervently discussed when vacancies for the U.S. Supreme Court appear likely.
Advocates for various social ideas or policies often wrangle heatedly over what litmus test, if any, the president ought to apply when nominating a new candidate for a spot on the Supreme Court.
Supreme Court Chief Justice Earl Warren was appointed under the impression that he was conservative but his tenure was marked by liberal dissents.
"[2] Professor Eugene Volokh believes that the legitimacy of such tests is a "tough question", and argues that they may undermine the fairness of the judiciary: Imagine a justice testifies under oath before the Senate about his views on (say) abortion, and later reaches a contrary decision [after carefully examining the arguments].
partisans on the relevant side will likely cry: They'll assume the statement made with an eye towards confirmation was a lie, rather than that the justice has genuinely changed his mind.