That requires a constitutional judgment. Answering strictly legal questions in a potential trial does not resolve the issue of the president’s accountability under the Constitution. Congress’s inquiry can and should be informed by an unfettered special counsel investigation, but it cannot depend on it.

Furthermore, narrowing impeachable offenses to include only violations of law may lead to a constitutional dead end. In opinions issued in 1973 and 2000, the Department of Justice’s Office of Legal Counsel has taken the position that a sitting president cannot be indicted while in office. If Mr. Mueller concludes that the president engaged in criminal conduct but follows O.L.C. opinions in declining to indict him, the president’s legal responsibility will not be adjudicated as long as he occupies the White House. On what basis would Congress then proceed to oust him from office under the legalistic conception of the impeachment power?

Many people assume that the special counsel will report to Congress on the evidence against the president. But the special counsel regulations, unlike the now defunct independent counsel statute, do not clearly mandate or authorize any such report from either the counsel or the deputy attorney general. Congress may be more likely to learn about Mr. Mueller’s work from publicly filed indictments and plea agreements.

And Congress cannot rely on the Mueller record alone. Even if Congress made impeachment a legal rather than political process, a president will be quick to argue that he is entitled to a fair adjudication of any criminal charge.

Moreover, the timetables for the two processes are not the same. Congress cannot responsibly defer its task for as long as it may take for lawyers to clash and courts to rule.

A Congress that was serious about meeting its responsibility would neither shirk nor rush a judgment about a president’s impeachable offenses. The House would structure an investigative and deliberative process that it would explain in clear terms to the public. As in 1974 in the Nixon impeachment process, the House Judiciary Committee would review and publish the best constitutional learning on what presidential misconduct rises to the level of “high crimes and misdemeanors.” It would proceed in the same spirit as its predecessor did when the 1974 committee said that “what is said here does not reflect any prejudgment” of the allegations but is “intended to be a review of the precedents and available interpretive materials, seeking general principles to guide the committee.” The Committee would then move to the investigative phase.