The clamor for President Donald Trump's impeachment is growing louder with every broken norm and breaking news alert. Peculiarly, it also seems like every article accusing him of an impeachable offense includes a reminder that he can be removed from office even if he didn't commit a crime.

As Charlie Savage put it for the New York Times: "As a practical matter, the Constitution's standards for impeachment and removal of a president—if he has committed 'treason, bribery, or other high crimes and misdemeanors'—are met by anything that a majority of the House and two-thirds of the Senate are willing to vote for." The writers on Lawfare are willing to set "questions of criminality aside" for "the far more significant issues" of whether President Trump violated his oath of office. Professor Laurence Tribe writes in the Washington Post that President Trump "could theoretically have been charged from the outset of this presidency" for, say, his "brazen defiance of the foreign emoluments clause." And on Vox, Julia Azari summarizes the impeachments of Presidents Andrew Johnson and Bill Clinton to conclude, persuasively, that “the main checks on presidential power are found in politics, not law.”

This consensus makes some sense. The Constitution authorizes only Congress to impeach and remove a president. No appellate court will ever double check whether Congress's rationale is legally justified. And when the House of Representatives voted to impeach President Johnson in 1868 and President Clinton in 1998, its articles of impeachment included several offenses that were not criminal. For example, one of the articles against President Johnson alleged that he fired his Secretary of War without legal authority.

But it would be a grave mistake to call for President Trump's impeachment if he hasn't committed a crime. First of all, impeaching a president for anything less than a crime or misdemeanor—one that was written down and in effect at the time of his alleged misconduct—is unconstitutional. Even more concerning, in an era of tit-for-tat partisanship, lowering the impeachment standard to “anything Congress thinks is wrong” is a recipe for dysfunctional government, one in which the House of one party could perpetually threaten to impeach the White House of another.

Starting with the first point, consider the constitutional standards for impeachment. Article I gives the House of Representatives "the sole Power of Impeachment" and the Senate "the sole Power to try all Impeachments." This language reflects eighteenth-century English practice, in which "impeachment" was a fancy word for initiating a criminal trial in Parliament. In the words of Sir William Blackstone, articles of impeachment were "a kind of bills of indictment," or a "prosecution of the already known and established law."

Article II, section 4, limits which established law an impeachment can prosecute: the "President, Vice President and all Civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." It's fair to infer from this sentence that an impeachable offense has to be a criminal one.

Finally, additional clauses limit Congress's power to create and punish criminal offenses. Article I, section 9, prohibits Congress from passing an "ex post facto Law"—a crime that applies retroactively to actions that were not criminal when they were committed. The Due Process Clause, meanwhile, prohibits Congress from outlawing vague, overbroad, or unwritten criminal offenses. Supreme Court decisions from the 1810s to the 2010s have overturned convictions for conduct that was immoral but not an offense that Congress had written down anywhere.

Putting this all together: an impeachment should begin only when the House of Representatives is convinced that a president has committed a crime—specifically an already-existing, written-down offense.

Of course, no one but the House can enforce this rule. And perhaps for that reason, the House has twice drafted articles of impeachment against the president that retroactively created new offenses or that imprecisely approximated existing ones.

When the House voted to impeach President Clinton, for example, it resolved that he had unlawfully "prevented, obstructed, and impeded the administration of justice." Although there are many obstruction-of-justice statutes, none of them are that broad. Section 1503 of the criminal code, for example, only punishes people who impede a judicial proceeding "corruptly, or by threats or force."

But the absence of judicial oversight is no excuse for Congress not to abide by its constitutional responsibilities. Like the president, members of Congress also take an oath to support and defend the Constitution. And the Constitution does not permit Congress to impeach and remove a president who hasn't violated a written, criminal offense that existed at the time of the alleged misconduct.

There is also a practical reason for Congress to limit its impeachment power to such crimes. You only need to look at the recent history of judicial nominations to see how escalating partisan warfare can backfire. In the 180 years before 1967, there was a strong and virtually untested norm against filibustering judicial nominees. But in the 50 years since, missile envy by both Democrats and Republicans has turned that norm into nuclear waste.

Impeachment is an even more dangerous weapon. If President Trump's critics take the impeachment of President Clinton as the Platonic ideal of what counts as an impeachable offense, the minimum "high crime" could quickly get out of hand. Future Congresses could define "obstruction of justice" as any interference they don't like of any proceeding that they do.

Or worse. It's not hard to imagine a Republican Congress in five years voting to impeach President Elizabeth Warren for "the high crime and misdemeanor of proposing a plan of socialized medicine."

At the end of the day, our Constitution has two avenues for prematurely removing a president. One, the Twenty-Fifth Amendment, begins when the most loyal members of his administration decide that he's incompetent. The other, impeachment, begins when over two-hundred elected representatives decide that he’s a criminal. Faced with a president who hints at his incompetence with every misspelled tweet, many people have nevertheless decided that the easier path to removal is the criminal one. And that has understandably led the president's critics to paint everything he's doing with a brush of criminality.

But criminality requires a crime—and the crimes of obstructing justice or committing espionage are far more demanding than a general distaste for the way the president carries himself with the FBI director or Russian ambassador.

For instance, even the criminal code's broadest definition of obstruction of justice, section 1505, requires proof not only that President Trump corruptly interfered with the "proper administration of the law," but also that he intended to interfere with a "proceeding." As others have pointed out, President Trump's admittedly selfish exercise of prosecutorial discretion is not necessarily incompatible with the "proper administration of the law." In addition, the relevant “proceeding” in this context probably cannot be an FBI investigation, as courts and even the U.S. Attorneys’ Manual have concluded that “investigations by the Federal Bureau of Investigation (FBI) are not section 1505 proceedings.”

These are just a sampling of the many limitations that arise when Congress, consistent with its constitutional responsibilities, drafts criminal offenses ahead of time instead of to suit a particular defendant. Beyond President Trump, these limitations are a vital part of our system of criminal justice. Ideally, they give people notice of what is against the law. They also ensure that prosecutors don't punish anyone for innocent conduct.

President Trump may be an unfit president. But even he deserves the Constitution's protection. At the very least, his successors do. Impeachment is a hazardous remedy. Its fallout will long outlast President Trump's seat in the Oval Office.

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