It is a commonplace among legal scholars that a judicial decision decided for political reasons, rather than legal ones, deserves criticism. It is also a commonplace, however, that historical arguments purporting to show that a precedent was primarily a political decision, rather than a legal one, have no place in the traditional doctrinal arguments that lawyers make before courts. Such arguments are understood to be appropriate for law-review journals and law-school classrooms but not for courts of law. But the justification for excluding them from the domain of legal argument is far from clear. This article uses a case in which the justices of the Supreme Court sparred over the propriety of offering a political or “extralegal” explanation of one of the Court’s precedents as an opportunity to examine the structure of such arguments and to consider the role they could play in precedential analysis. It first shows the way in which arguments that purport to undermine the precedential authority of a past decision by offering a political explanation of it are analogous to arguments lawyers make when impeaching witnesses at trial. It then argues that such “impeaching arguments” may be a legitimate and useful method of evaluating the weight of precedent but that whether they are or not depends on one’s theory of precedent. The article’s ambition is less to advocate the use of arguments impeaching precedent than to identify the assumptions of such arguments and to trace out their implications for legal reasoning.