Ever since the special counsel began his Russia investigation, President Donald Trump, backed by his Justice Department, took the position that a sitting president could not be indicted and that impeachment is the only remedy if he committed a crime such as obstruction of justice. But now, a May 19 opinion from the Justice Department's Office of Legal Counsel seeks to close off that avenue as well, by making it impossible as a practical matter for Congress to obtain the evidence it needs for impeachment. Here's why.

That opinion concludes that the president has the constitutional right to prevent any of his current or former advisers from testifying before Congress, no matter what the subject is, or why Congress wants the testimony. On its face, that opinion cannot stand for three reasons, but its most important defect is that it would effectively delete the remedy of impeachment from the Constitution.

First, the principal basis for the opinion is that the president's lawyers have always said that the president's advisers are immune from congressional subpoenas, which is hardly a surprise, but is surely not a legal reason to sustain their views. Second, the opinion notes that there are no appellate decisions directly on point, although the Supreme Court rejected a similar absolutist claim involving the White House tapes of President Richard Nixon. One reason is that in most of these confrontations, the parties reach a compromise, as the opinion recognizes, either by providing at least some of the requested information or by having the witness be available for informal access, short of formal testimony in an open session. For Trump, apparently compromise is a four-letter word that is not in his vocabulary.

Third, as the OLC opinion finally admits on page 10, there is one district court decision on point. House of Representatives v. Harriet Miers rejected the claim that the former White House counsel could refuse to appear and decline to answer any questions. An appeal was taken, but not pressed, and eventually the House and the new administration resolved their differences and the appeal was dismissed.

The courts are currently wrestling with a number of subpoena enforcement cases in which the House is arguing, successfully so far, that Congress must have the power to issue subpoenas to investigate as a basis to enact laws. The House should win most of those cases, although it may have more difficulty if it insists on having individuals who now work directly for the president testify about the basis for his policy decisions on, for example, Iran or the detention of asylum seekers, especially if there are others in the executive branch who can provide Congress the information that it needs.

But whatever happens with subpoenas to White House staff in other cases, impeachment is different for several important reasons. Unlike Congress' implied power to investigate, or the president's implied power of executive privilege, impeachment is expressly provided for in the Constitution. Second, if a president commits a crime, such as obstruction of justice, it is likely that the main and perhaps only witnesses will be his closest aides, all of whom would be barred from appearing before the House Judiciary Committee, unless the president waived his privilege. Third, assuming the Justice Department is correct, and that a sitting president cannot be indicted, if he can also stop all the key witnesses from testifying, the House might not have enough evidence to impeach, and the Senate would have an almost impossible burden to obtain the votes of two-thirds of the Senate to convict.

Two examples from not-too-distant history show how pernicious this absolutist view of presidential power is. If the OLC is correct, then Nixon could have stopped his former counsel John Dean from testifying before Senate initially, and later the House, about the president's efforts to obstruct justice. Similarly, President Bill Clinton could have prevented former White House intern Monica Lewinsky from telling Congress about her sexual encounters with the president in the Oval Office.

The president has the right to prevent Congress from routinely dragging in White House staff to testify, or insisting on access to every paper or email in the White House system, if he is to carry out his constitutional duties. But Congress has its own constitutional responsibilities, including that of impeachment. Unless the courts are prepared to declare that presidential accountability is at an end, and impeachment is to become a dead letter, the House must be able to hear from all witnesses who have relevant information on whether the president has committed an impeachable offense, including his former counsel and others who work for the president.

Alan B. Morrison is the Lerner Family Associate Dean for Public Interest and Public Service at the George Washington University Law School. Morrison previously worked for the Public Citizen Litigation Group, which he co-founded with Ralph Nader in 1972 and directed for over 25 years.