President Trump in the Oval Office on Wednesday. (Evan Vucci/AP)

George T. Conway III is a lawyer in New York. Neal Katyal, a law professor at Georgetown University, previously served as the acting solicitor general of the United States.

Much ink has been spilled about whether President Trump committed a criminal and impeachable offense by obstructing justice. That question deserves extensive debate, but another critical question — the ultimate question, really — is not whether he committed a crime but whether he is even fit for office in the first place. And that question — the heart of an impeachment inquiry — turns upon whether the president abuses his power and demonstrates an unfitness to serve under the defining principles of our Constitution.

On Tuesday, Trump gave us direct evidence of his contempt toward the most foundational precept of our democracy — that no person, not even the president, is above the law. He filed a brief in the nation’s second-most-important court that takes the position that Congress cannot investigate the president, except possibly in impeachment proceedings. It’s a spectacularly anti-constitutional brief, and anyone who harbors such attitudes toward our Constitution’s architecture is not fit for office. Trump’s brief is nothing if not an invitation to commencing impeachment proceedings that, for reasons set out in the Mueller report, should have already commenced.

Legislators treated the investigation into Richard Nixon with the seriousness it deserved. Inquiries into President Trump fall far short. (Video: The Washington Post)

The case involves a House committee’s efforts to follow up on the testimony of Trump’s now-incarcerated former attorney, Michael Cohen, that Trump had allegedly committed financial and tax fraud, and allegedly paid off paramours in violation of campaign finance laws. The House Committee on Oversight and Reform subpoenaed Trump’s accountants in mid-April for relevant documents, and Trump tried to block the move, only to be sternly rebuked in mid-May by a federal judge in Washington.

The appeals brief filed Monday by Trump attacks that decision. But to describe Trump’s brief is to refute it. He argues that Congress is “trying to prove that the President broke the law” and that that’s something Congress can’t do, because it’s “an exercise of law enforcement authority that the Constitution reserves to the executive branch.”

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But in fact, Congress investigates lawbreaking, and potential lawbreaking, all the time. Mobsters, fraudsters, government employees, small companies, big companies — like it or not, all types of people and businesses get subpoenaed from time to time so that Congress can figure out whether current laws are effective, whether new laws are needed, whether sufficient governmental resources are being devoted to the task, whether more disclosure to the government or the public is required, or greater penalties, and so on.

President Trump on June 10 said, “You can’t impeach somebody when there’s never been anything done wrong.” (Video: The Washington Post)

To this, Trump’s brief complains that “Congress could always make this (non-falsifiable) argument” to justify any investigation. But that’s simply the result of the fact that, as the district court explained, Congress’s “power to investigate is deeply rooted in the nation’s history.” Congress, relying on English parliamentary tradition, has performed this function since the founding.

To accept Trump’s argument to the contrary — to say Congress can’t look into matters that might involve crimes — would in many cases gut Congress’s ability to gain information it needs to legislate. And perversely, in Trump’s case, it makes a virtue of the fact that he has been accused of committing crimes.

Which brings us to the main point: England’s King George III was above the law, but the founders of our republic wanted a system that would divide power and have the branches check one another. The idea that only the president can investigate the president is an argument for autocrats, not Americans.

Trump says “trust me,” but that was exactly the argument the founders rebelled against. They knew that public officials would not always be angels, and that power had to be checked and dispersed. As James Madison put it in Federalist No. 51, “It may be a reflection on human nature, that such devices should be necessary to control the abuses of government.”

The only redeeming quality of Trump’s legal brief is its seeming, grudging acknowledgment that Congress’s powers might be greater in an impeachment proceeding. That has things only half-right. Yes, Congress could investigate Trump’s finances in an impeachment proceeding, but it can do so without launching the formal process of impeachment.

That said, Trump’s brief can be construed as an invitation to commence impeachment proceedings. In those proceedings, Trump’s attitudes toward our Constitution’s checks and balances, in addition to evidence of obstruction of justice, must play a key role. Indeed, the third article of impeachment against President Richard M. Nixon, adopted by the House Judiciary Committee in 1974, charged him with defying lawful subpoenas issued by the House Judiciary Committee.

Not only has Trump done that, but he has also demonized judges who disagree with him and insulted the press (despite its constitutional status) for calling him to account. Other leaders around the world may behave this way, but these are not proper actions of a president of the United States. What makes the United States exceptional is its commitment to its constitutional architecture, particularly divided powers.

For the past three decades, many constitutional law classes have begun with Nixon’s breathtaking statement to David Frost in May 1977: “Well, when the president does it, that means that it is not illegal.” Generations of students have gasped, shocked that a former president could say such a thing. This time, it’s not a former president but a sitting one. Every principle behind the rule of law requires the commencement of a process now to make this president a former one.

Read more:

Harry Litman: Can Democrats break the Republican hammerlock on impeachment?

Max Boot: Here are seven reasons Trump should be impeached

Fred Hiatt: We knew who Trump was but elected him anyway. We can’t impeach him for that.

David Von Drehle: Trump is counting on impeachment

Marc A. Thiessen: Americans don’t want Congress to impeach, but Democrats aren’t listening