The extraordinary letter from White House Counsel Pat Cipollone to House Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE outlines President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s numerous complaints about the impeachment process. At least as to the argument that the current inquiry is invalid because there has been no formal vote by the House of Representatives, there should be little doubt that Cipollone is wrong. No such vote is necessary because the Constitution dictates no fixed process for impeachment investigations or trials. This know from the U.S. Supreme Court’s rejection of Judge Walter L. Nixon’s challenge to his impeachment more than twenty years ago.

Nixon was convicted on two counts of making false statements before a grand jury. The grand jury investigation followed reports that Nixon had “accepted a gratuity from a Mississippi businessman in exchange for asking a local district attorney to halt the prosecution of the businessman’s son.” Even after he was sentenced, Nixon refused to resign his office as a United States District Court Judge. No resolution by the House authorized the subsequent investigation into Nixon. The House ultimately adopted three articles of impeachment, charging Nixon with giving false testimony and with “bringing disrepute on the Federal Judiciary,” and the Senate convicted him on the first two.

The Supreme Court’s involvement stemmed from a lawsuit Nixon filed in an effort to invalidate his impeachment. He argued that the Senate’s process was defective because his conviction did not result from the kind of trial that resembled a judicial proceeding.

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Nixon pinned his hopes on the constitutional text authorizing the Senate to “try” all impeachments. But the court noted that the word “try” has multiple meanings — and it would not choose among them. Rather, the court concluded that the framers had assigned to the Senate the exclusive discretion to determine how best to adjudicate charges contained in duly adopted articles of impeachment. And, where the Constitution clearly makes such an assignment, it was not for the judiciary to second-guess the Senate’s judgment about how to proceed in a particular case.

The court in Nixon v. United States accordingly confirmed that, when the Constitution expressly assigns to one department of government – or, in the case of impeachment, one part of one department – the responsibility for a particular function, that department can decide for itself how to fulfill its responsibility. While Nixon concerned the Senate’s role in the impeachment process, the same reasoning applies to the House and the constitutional mandate that it “shall have the sole Power of Impeachment.”

If the House has the sole discretion to adopt articles of impeachment, it follows that it has the discretion to determine how to launch and conduct any preceding inquiry and investigation. This conclusion finds support in another textual mandate in the constitution: that each House “may determine the Rules of its Proceedings.” Notably, there is no impeachment exception to this provision.

That impeachment inquiries and investigations are left to the House’s discretion makes sense. The House is the most democratically representative department of the federal government, and the framers reasonably could have concluded that no impeachment investigation would occur unless a majority of the House membership approved. Given the potential political obstacles to assembling a majority of the House to act on any matter, much less an issue as fraught as impeachment, the framers likely understood that the power to investigate and, potentially, adopt articles of impeachment would be used cautiously.

Concurring in the Nixon court’s judgment, Justice David Souter nonetheless suggested that an entirely arbitrary process by the Senate would raise constitutional concerns—if, for example, the Senate decided whether to convict based upon a coin toss. Similar reasoning could be applied to the House: If a majority decided to adopt articles of impeachment absent any inquiry whatsoever into the underlying circumstances, it is not clear the House’s judgment should be respected.

But it cannot be right that the impeachment process or its result should be deemed arbitrary simply because the president says so. In light of the publicly available evidence of the president’s actions in respect to Ukraine, moreover, there is no good argument that the House has undertaken the current investigation on a whim. Indeed, the quantity of evidence suggests that, at this point, it would be constitutionally irresponsible for the House not to continue to pursue its investigation into whether the president’s conduct qualifies as a high crime or misdemeanor.

Lawrence Friedman teaches constitutional law at New England Law | Boston and is the author, most recently, of the second edition of “Modern Constitutional Law.”