President Trump talks to members of the media at the White House on Jan. 14. (Jabin Botsford/The Washington Post)

Neal K. Katyal was acting U.S. solicitor general from 2010 to 2011. Michael V. Hayden was director of the National Security Agency from 1999 to 2005 and the Central Intelligence Agency from 2006 to 2009.

We have been exceptionally reluctant to call for the impeachment of President Trump. Impeachment runs the risk of undoing an election and dividing the nation. But there is a big difference between calling for impeachment and calling for an investigation into whether impeachment is appropriate.

After careful review of the articles of impeachment for President Richard M. Nixon, we now believe it is appropriate for the House of Representatives to begin the process by launching an impeachment investigation. No legislator should rush to judgment one way or the other. The process should be designed to uncover the facts.

But if the facts show that Trump ordered or participated in the commission of crimes, in particular crimes that may have allowed him to win the election, we think it incontrovertible that impeachment would be warranted. So too, if the facts show that Trump ordered his personal attorney Michael Cohen to lie to Congress, as BuzzFeed reported Thursday night in an article that the special counsel is now disputing, he would be guilty of an impeachable offense. One of the Nixon articles was for “approving, condoning . . . and counselling witnesses with respect to the giving of . . . false or misleading testimony in duly instituted judicial and congressional proceedings.”

Separately, if evidence shows that the president has acted as an agent of a foreign power, impeachment would be a proper remedy.

Things might be different if there were a guarantee that Trump would face the consequences of any criminal activity. But many of the president’s defenders have embraced a broad view of executive power, including a radical view that the president cannot generally obstruct justice, as well as the notion that a sitting president cannot be indicted.

Every serious scholar who believes a sitting president cannot be indicted couples it with the need for impeachment in such circumstances — otherwise a president would be above the law. So it follows, from Trump’s own defenders, that the process of investigating whether impeachment is warranted must begin. Trump seeks immunity from internal criminal investigation — but if he is immune from such a process, he must then face a separate external one: by the House of Representatives in impeachment proceedings, as our founders specified in Article I, Section 2, of the Constitution.

On the intelligence side, a host of revelations simply in the past few days should cause all Americans concern. A president meeting with Russian President Vladi­mir Putin and taking control of all notes, as reported by The Post? An FBI investigation into whether a sitting president is a compromised Russian asset, as the New York Times reported? And then all the deceptions, denials and distractions by Trump, including shutting down the government based on a rather thin claim of national security. Congress must fulfill its constitutional duty to protect our national security.

The recent statement by the special counsel’s office disputing the BuzzFeed article itself highlights the need for a congressional investigation. The BuzzFeed article alleges that Trump ordered Cohen to lie to Congress. Congress of course is the entity with the most at stake when it comes to such a crime. No entity is better poised to find the truth and reveal the facts to the American people.

The president and his advisers have tried already to block the government from investigating these questions. They have criticized special counsel Robert S. Mueller III as engaged in a “witch hunt” and attacked Cohen as a “rat” and a liar. Trump’s lawyers have threatened to assert executive privilege to block answers to Mueller’s questions and threatened to block release of Mueller’s report. A senator has indicated he won’t pursue the interpreter’s notes due to executive privilege. All of this points to a severe danger that, absent an investigation into impeachment, there will be no process to ferret out the truth and report it to the American people.

It was one thing when a group of professionals were at the helm of the executive branch’s national security establishment. But nearly all of them are gone. Former national security adviser H.R. McMaster is gone. There is no Rex Tillerson at the State Department, nor Nikki Haley at the United Nations. John F. Kelly has left the White House, and Jim Mattis has resigned as secretary of defense. Each served as something of a check to a self-interested and possibly compromised president. Congress is all the American people have left.

Both of us have been critical of the president. But we fervently hope and pray that an impeachment investigation would come back with no evidence of crimes or conspiracy. To find such evidence would rip at the fabric of our nation and force a wrenching process upon it. There is only one thing worse than knowing the answers to these questions, and that is not knowing them.

Impeachment skeptics say it will be impossible in this hyperpartisan age to convict a president, even a guilty one, due to the constitutional requirement that two-thirds of the Senate must agree. But that requirement is a feature, not a bug, of our system. Our founders, in their wisdom, gave us an impeachment process that permits the House to diligently investigate the questions surrounding an individual without the fear that the investigation alone, or a political frenzy, would be enough to convict a president.

We don’t think the questions are ultimately lawyerly ones, thus our joint authorship today. Instead, they involve simply asking one thing: Is Trump obeying his oath to support and defend the Constitution and take care that the laws be faithfully executed? The time has come for Congress to fulfill its constitutional duty and seek the answer.