J. Scott Applewhite / AP

It’s tempting to think of the impeachable offense as a static construct. Either the president did a bad thing, or he didn’t. Either it was sufficiently bad as to be impeachable, or it was not.

But this can’t be correct, either as a matter of common sense or as a matter of constitutional law. As House Democrats roll forward with public impeachment hearings, and as Senate Republicans build out their defense strategy, careful onlookers will come to find that the president’s conduct isn’t a subject that can be studied in isolation. Congress’s past and present behavior, too, shape the standard of what qualifies as an impeachable offense.

Article II, Section 4 of the Constitution provides the bare silhouette of the impeachable offense: “Treason, Bribery, or other high Crimes and Misdemeanors.” Congress is tasked with filling in that blurry shape. Congress can do this in two ways. The obvious way is by vote. This is what then-Representative Gerald Ford was getting at when, in 1970, he famously defined the impeachable offense as “whatever a majority of the House of Representatives considers it to be.” As a procedural matter, at least, this is true. By voting, the House throws its weight behind a particular understanding of the impeachable offense. After impeachment, the Senate can then reinforce that judgment by voting to convict and remove the president by the required two-thirds supermajority, or not.

But a second, more quotidian process unfolds outside of and apart from the impeachment process. In its day-to-day work, every time Congress excuses or endorses executive conduct, even implicitly by not taking action, Congress makes it that much more difficult—and constitutionally questionable—to turn around and impeach a president for it or similar conduct in the future.

In some cases, particularly in realms where the executive and legislative branches share power, Congress can—and occasionally does—draw clear lines. The classic example is where Congress sets a limit on the president’s initiation of force; the president violates that at his peril. This doesn’t mean he risks impeachment any time he oversteps a congressional limit. Congress has a diverse menu of options when it comes to molding executive behavior, from committee hearings to appropriations threats. The point is, through toleration and admonishment, by acting or failing to act, Congress has the power to shape the outer bounds of what can be reasonably understood as acceptable presidential conduct—and as a consequence, what can be reasonably understood as an impeachable offense.

This is not just an observation about simple fairness. The Constitution actually contains guardrails against congressional hypocrisy and fickleness. In his influential 69-page handbook on impeachment, published shortly before Richard Nixon’s resignation, the late, great constitutional scholar Charles Black Jr. explores these guardrails, starting his analysis with the only record we have of the Founders’ discussion of the phrase other high Crimes and Misdemeanors.

On September 8, 1787, a little more than a week before the signing of the Constitution, George Mason (a key critical voice at the Constitutional Convention, and one of three delegates who ultimately refused to sign) took issue with the draft impeachment provision, which at the time provided for impeaching the president only for treason or bribery. Mason argued that this was too narrow: For instance, “attempts to subvert the Constitution may not be Treason.” The compromise language that the Founders settled on was the addition of the phrase other high Crimes and Misdemeanors.

Black stresses the importance of Mason’s reasons for his view. Mason explained, “As bills of attainder which have saved the British Constitution are forbidden, it is the more necessary to extend: the power of impeachments.” What did he mean by this? Bills of attainder were legislative acts that, without trial, criminalized the past conduct of specific persons or groups and subjected them to punishment. Britain had relied on them to remove people from public office, and without them America would need to define the impeachment power expansively enough to capture the full range of extreme presidential misconduct.

Although Mason specifically referred only to bills of attainder, Black offers an additional insight: The Founders abhorred bills of attainder for much the same reason they abhorred another sort of law, with which they were closely associated: ex post facto laws, which criminalize past conduct. The Founders included in the Constitution two separate clauses forbidding both the federal government and the states from passing both types of laws.

Black reasons that Mason’s remark about bills of attainder demonstrates that the Founders must have recognized that Congress didn’t have the power to eject the president from office and into eternal historical ignominy for conduct that he arguably didn’t know was wrong when he did it. His basic insight here is that Congress can’t do to the president what it is forbidden from doing to regular citizens, and he uses Mason’s comment to show that the Founders very much understood that. For that reason, in Black’s view, the only way to be true to the “spirit and equity of the bill of attainder and ex post facto clauses” in the impeachment context is to “treat as impeachable those offenses, and only those, that a reasonable man might anticipate would be thought abusive and wrong, without reference to partisan politics or differences of opinion on policy.”

The logic is straightforward enough, and the proposition easy enough to accept. But the implications are enormous. Consider two.

The first and more practical observation to be made here is that to warrant impeachment, it may not be enough for the president to do something objectively abusive, such as pursue his own personal interests at the expense of the country’s. That may not be impeachable by itself if Congress has behaved in a way that creates an open question of whether such conduct is out of bounds.

Perhaps the best example of this comes from the emoluments lawsuits against President Donald Trump that have been going on for years, one of which was brought by congressional Democrats. The suits allege that Trump has violated the domestic and foreign emoluments clauses by failing to put his assets in a blind trust and by continuing to profit from foreign and state entities—which can try to curry his favor by, for instance, buying his merchandise and renting rooms at his hotels. Whether or not you agree with the claim, and even if the Democrat-controlled House could now agree that his conduct violates the clauses, the problem with treating Trump’s “emoluments” as a basis for impeachment is that the Department of Justice has filed brief after brief defending Trump’s right to carry on. If Congress wanted to do something about this, it could have. And for years it chose not to. That’s a fact that cuts against impeachment even now, after political power has shifted in the House, because whether action is justified or not, the delay renders the issue nakedly partisan.

It’s not that Congress’s failure to enact a law about the president’s ability to profit from his office says anything conclusive about whether the Constitution in fact prohibits the president from engaging in this kind of behavior. But impeachment is a process that requires our elected representatives to fairly consider whether the president has committed what a reasonable man would understand to be a high crime or misdemeanor. It would be hard for Congress to vote that the president did this, given that Congress has allowed the Justice Department to spend years defending his right to do it. Understood in this light, a proper impeachment inquiry will tend to zero in on instances of alleged presidential misconduct for which the president cannot readily hide behind the acquiescence of Congress, or of other institutional actors. This, and not just political strategy, might help explain why House Speaker Nancy Pelosi has reportedly sought to limit the House’s ongoing impeachment probe to allegations relating to Trump’s call with Ukraine.

The emoluments example points to a second, more fundamental conclusion to be derived from Black’s proposal that Congress can punish only what a president could be reasonably expected to know is wrong, based in part on the cues he gets from Congress: If congressional judgment is the unavoidable filter through which America as a nation discerns presidential wrong, Congress’s refusal to responsibly exercise that judgment is guaranteed to distort both the country’s perception of presidential power and how presidents wield it. Extreme congressional partisanship thus poses a threat not only to constitutional norms but also to the shape and content of constitutional law.

This is not how we typically think about constitutional law. Even when lamenting their demise, it’s common to treat norms as the redheaded stepchildren of the constitutional landscape—they are soft, squishy, and less important than hard “law.” But Black’s analysis suggests that the law of impeachment builds directly on our collective conception of what good faith and reasonableness require. On this view, norms do not sit separate and apart from law; they are embedded in it.

Impeachment skeptics are fond of cautioning against a rush to judgment that could undermine the legitimacy of electoral outcomes. But the norm-law connection helps explain why Congress’s willingness to tolerate intolerable behavior from the White House can be so destructive. The result isn’t just damage to our social codes and political conventions; over time, that toleration can erode our ability to ever sanction abuses of power.

The stakes of the current impeachment saga are heightened by the potential remaking of an old taboo most famously reaffirmed during Watergate, on the abuse of executive authority to investigate political opponents, this time with the additional complexity of having implicated a foreign nation. To those favoring impeachment, this makes the abuse worse, as in their view it demonstrates a subordination of America’s interests abroad to the president’s personal gain.

But the strategy on the right suggests that the foreign twist has in effect created a maze of complicated fact questions—such as whether President Trump exerted any pressure when he brought up the Bidens, and whether Ukrainian President Volodymyr Zelensky was aware of a hold on U.S. security assistance at the time of the call—that could be deployed to clear the president, at least formally, of impeachment-level wrongdoing. These two approaches to the impeachment inquiry are likely irreconcilable, and the partisanship on display in what the members of Congress say about the president’s conduct, and the logic with which they choose to accuse or excuse, could prove as important to our constitutional culture as how they ultimately vote.

Impeachment—half legal prescription, half political process—is the minotaur of constitutional law. It’s ugly and lives trapped in a confusing labyrinth of bad takes and shameless partisanship. Exacerbating this is the fact that we haven’t had many impeachments, so every move Congress makes in connection with the process ends up taking on outsize significance. Two decades after the impeachment of Bill Clinton, and a century and a half after the impeachment of Andrew Johnson, the American public is still, incredibly, picking apart those proceedings and attempting to draw lessons from the mistakes that were made and the disasters that were avoided.

It’s no coincidence that from this lean literature America tends to recognize as heroes the handful of representatives and senators who voted in defiance of their party and in adherence to their oath. When our elected representatives decide, myopically, to support or oppose a president based primarily on party affiliation, they may believe themselves to be saving or torching a single presidency, but in truth they are shaping, for the ages, our collective take on what reasonable presidential behavior looks like. They are bequeathing to generations to come an extremely concrete set of pronouncements about how to approach the most difficult of constitutional balancing acts: between protecting the integrity of the democratic process and protecting the country and the international community against what James Madison described as “the incapacity, negligence or perfidy” of the person occupying what is in our age the most powerful office on Earth.

It’s not legislation. But make no mistake about it: When it comes to impeachment, Congress is always making (constitutional) law.

This story is part of the project “The Battle for the Constitution,” in partnership with the National Constitution Center.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.