It later emerged that there was indeed a connection between the burglary and Nixon’s White House. Whether or not Nixon and his team had authorized it, they had arranged to pay “hush money” to the burglars, and his White House tried to enlist the C.I.A. to help counteract the F.B.I.’s investigation into the burglary. At its heart, the Watergate scandal is a tale of a coverup—not the worst thing in the world, but not good. Impeachable? We will get to that.

Formally, impeachment proceedings start with the drafting of “articles of impeachment,” which are written and voted on by the designated committee within the House of Representatives. If the committee votes in favor, the articles proceed to a vote in the full House. If the House votes to impeach (a bare majority is required), the issue goes to the Senate, which votes whether to convict (a two-thirds majority is required).

In the case of Nixon, several articles received serious consideration by the Judiciary Committee of the House of Representatives. Because of his resignation, there was no vote in the full House.

The I.R.S. ruled that, in his first years in office, Nixon underpaid his taxes by a total of more than four hundred thousand dollars. Note that he did that as President, not as a private citizen. That’s a lot of money (especially if you adjust for inflation). You could argue that such a large underpayment, from a President with access to the finest legal advice, was a product of something much worse than mere negligence.

But tax evasion isn’t an impeachable offense. It’s not an abuse of official authority. It’s in a wholly different category from the high crimes and misdemeanors that concerned Madison and Hamilton, and that would justify impeachment. The vote against proceeding was 26–12; it should have been 38–0. (To the twelve Democrats who voted in favor: not good.)

The House and Senate are fiercely protective of their own prerogatives, not least when they are seeking materials from the executive branch. They do not like to be thwarted. For its part, the executive branch is deeply suspicious of investigations, thinking that they are politically motivated. Its officials do not love to hand over documents. They are fiercely protective of their own deliberative processes, and that is true whether the President is Republican or Democratic.

If White House officials speak to one another behind closed doors, the President’s lawyers will not want Congress or the public to know what was said. And, if the President himself is involved in the conversations, the executive branch will vigorously resist disclosure. There is a legitimate reason for that resistance: if people are to be candid, and to venture their arguments and express concerns, it is important for them to know that they can speak in confidence. With this point in mind, the executive branch will probably even claim that the Constitution itself protects the President’s right to keep things confidential.

In 1974, the Supreme Court agreed with that claim, ruling that the President has a presumptive right not to disclose his conversations. (The case had a good name: United States v. Nixon.) The Court emphasized the need for candor. In its view, a Presidency cannot function if the boss and his advisers are unable to keep their discussions private. At the same time, the Court ruled that the presumption could be overcome, and so disclosure could be mandated, by a demonstrated, specific need for evidence in a pending criminal trial. And so the United States won, not Nixon.

Nixon refused to comply with the Committee’s subpoenas. By a narrow vote of 21–17, the Judiciary Committee found, in that very refusal, a basis for impeachment. (Democrats voted in favor, 19–2; Republicans voted against, 2–15.) It charged that he failed to comply with the Committee’s subpoenas, even though they called for information that was needed to resolve “fundamental, factual questions relating to Presidential direction, knowledge or approval of actions demonstrated by other evidence to be substantial grounds for impeachment of the President.”

That seems pretty grave. In a way, it certainly is. But, under the constitutional framework, disobeying a subpoena is not necessarily an impeachable offense. Everything depends on what the subpoena is for. On one hand, the failure to respond to a subpoena that stems from suspicion of independently impeachable actions is hardly as grave as those actions. Maybe the suspicions are unfounded. Maybe the actions never took place. Maybe the President thinks that he is being subjected to witch hunt, or at least a politically motivated effort to damage him.

On the other hand, the Constitution gives the House the authority to investigate whether impeachable wrongdoing has occurred. If the President declines to coöperate with a lawful investigation, and if he has no good-faith argument that he is legally entitled to do so, there is a strong argument that he has committed a misdemeanor—not in the criminal sense but within the meaning of the impeachment clause. And this, in fact, appears to be the claim in the article of impeachment against Nixon.

If the President refuses to coöperate with a lawful investigation into whether he has done something impeachable, he is abusing his power. Has he committed an impeachment offense? The answer isn’t entirely obvious, but he probably has.

A separate article referred to the Watergate controversy itself—to the unlawful entry into the headquarters of the Democratic National Committee “for the purpose of securing political intelligence.” According to the article, Nixon had been behind an elaborate conspiracy to cover up the crime by lying to investigators, withholding evidence, interfering with lawful investigations, paying “hush money,” and trying to misuse the C.I.A.

The Judiciary Committee voted in favor of this article by a whopping margin of 27–11. But the margin conceals a big partisan difference. All twenty-one Democrats on the Committee supported it; only six of seventeen Republicans did so.

This article didn’t get to the heart of what impeachment is all about, but it did establish an impeachable offense. The President’s campaign committee committed unlawful acts to promote his own reëlection (a patent violation of democratic norms, itself impeachable if undertaken at the President’s direction). When those unlawful acts came to light, the President did not disclose them, as he should have, but instead used official power, sometimes in violation of the law, to prevent people from knowing about them. The sheer accumulation of charges makes that argument compelling.

Nixon was separately charged with offenses that do fall within the core of the impeachment clause. Here, the vote of the Judiciary Committee was the same as for the preceding article, with a similarly partisan breakdown. If we assume that the articles accurately stated the facts, the vote should have been unanimous; partisanship prevented many Republicans from doing their constitutional duty.

The three strongest charges said that Nixon tried to obtain information from the I.R.S. to produce income audits, in violation of people’s constitutional rights; that he misused the F.B.I. and the Secret Service, again in violation of people’s constitutional rights, to engage in electronic surveillance; and that he maintained a secret investigation unit, within the office of the President, to use the C.I.A. to engage in unlawful covert activities.

It’s tough to argue about those three. Indeed, they get at the heart of the concerns expressed during the ratification debates back in 1787, when the impeachment provision was directly linked with the preservation of liberty. If a President uses the apparatus of government in an unlawful way, to compromise democratic processes and to invade constitutional rights, we come to the heart of what the impeachment provision is all about.

In the two actual impeachments of American Presidents, no impeachable offense was committed. In a sense, the founding document worked: the Senate refused to convict. Still, the nation was badly served.

More than two decades later, the impeachment of Bill Clinton is almost incomprehensible, at least if it is explored in light of the debates in the late eighteenth century. Given the concerns of the framers and the ratifiers, you would have to work really hard to make even a minimally plausible argument that he committed an impeachable offense. But Clinton gave his political opponents an opening, and they were willing to work really hard.