The Case for Impeachment by Allan J. Lichtman William Morrow/Dey St., 290 pp., $24.99 Impeachment: A Citizen’s Guide by Cass R. Sunstein Harvard University Press, 199 pp., $7.95 (paper) (to be published on October 2)

1.

As more and more evidence of collusion between Donald Trump’s presidential campaign and Russia has come to light, the analogy to Watergate has grown ever stronger. In both cases, a burglary of the Democratic National Committee, undertaken to influence the outcome of an election, ignited a burgeoning scandal. Trump’s firing of FBI Director James Comey and warnings to Special Prosecutor Robert Mueller conjure President Nixon’s Saturday Night Massacre. Trump echoes Nixon in raging against leaks and decrying the investigation of his office as a “witch hunt.” There was brief excitement about Trump’s suggestion that he too might have a presidential taping system (though this seems to have been only a bluff).

Our lexicon for political scandal derives largely from Watergate, so it is almost impossible to discuss executive branch misdeeds without referring to it. Phrases like “It’s not the crime, it’s the cover-up” and “What did the President know and when did he know it?” are embedded in our national consciousness. We all know where the Watergate comparison leads, of course: to constitutional crisis and impeachment, the fate Nixon evaded only by his resignation. It is comforting to Trump’s opponents to think of this outcome as inevitable, but of course it is not. Whether or not it is “worse than Watergate,” the Trump-Russia scandal differs from it in ways that bear directly on how impeachment might serve as a remedy today.

Because it has been used so rarely, and because it is a power entrusted to Congress, not the courts, impeachment as a legal process is poorly understood. There are no judicial opinions that create precedents for how and when to proceed with it. Past cases are subject to competing and often contradictory interpretations. Some might even be tempted to argue that because impeachment is ultimately political, it cannot be considered in legal terms at all.

That extreme view cannot be right. Impeachment must be a legal procedure because it derives from specific constitutional directives. The impeachment clauses of the Constitution are subject to interpretation, like all language, legal or otherwise, but they function as law. Members of Congress have a sworn legal duty to apply the Constitution correctly—including when they are considering impeachment. Calling for the impeachment of Supreme Court Justice William O. Douglas in 1970, then Congressman Gerald Ford asserted that “an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” That statement ought to be taken as a description of political reality, not a prescription that Congress may choose to treat any conduct as impeachable.

The legal limits of the impeachment power are subject to debate.1 Yet it is clear both historically and logically that impeachment was designed to deal with abuses committed while in office, not prior crimes. Any wrongdoing of Trump’s before he assumed the presidency must be considered separately from offenses he…