IS MATTHEW G. WHITAKER the legitimate acting attorney general? From approximately the second President Trump ousted Attorney General Jeff Sessions and tapped Mr. Whitaker to temporarily exercise the office’s vast authority, legal experts have sparred over whether Mr. Trump can unilaterally elevate someone from a role that does not require Senate confirmation to one that does. But regardless of whether the promotion is legal, it is very clear that it is unwise. Mr. Whitaker is unfit for the job.

Several prominent legal scholars point out that the Constitution demands that “principal officers” of the United States must undergo Senate confirmation. A 19th-century Supreme Court case suggests there may be limited room for temporary fill-ins, but Mr. Whitaker’s appointment is hardly so temporary; he could serve for most of the rest of Mr. Trump’s first term. Even if Mr. Whitaker’s promotion is constitutional, Congress passed a law governing Justice Department succession that also seems to prohibit Mr. Whitaker’s ascent. The department has a capable, Senate-confirmed deputy attorney general in Rod J. Rosenstein; he should be running the department in the absence of a permanent replacement.

The Senate above all should be offended by the president’s end run around its authority. Majority Leader Mitch McConnell (R-Ky.) should demand hearings and consider filing a lawsuit. Instead, he is helping to establish a troubling precedent, saying only that he expects Mr. Whitaker to be a “very interim AG.” Yet no random official should be endowed with all the powers of an office as powerful as attorney general, meant for a Senate-vetted individual, even for a relatively short time.

And Mr. Whitaker is worse than random. It took less than 24 hours for material to emerge suggesting he could not survive even a rudimentary vetting.

First, there are Mr. Whitaker’s statements criticizing the Russia probe of special counsel Robert S. Mueller III. At the least, they require him to consult Justice Department ethics counsel about whether he can oversee the inquiry with a plausible appearance of evenhandedness. He will do immediate and lasting harm to the Justice Department’s reputation, and to the nation, if he assumes the role of president’s personal henchman and impedes the Mueller probe.

Then there is Mr. Whitaker’s connection to a defunct patent promotion company the Federal Trade Commission called “an invention-promotion scam that has bilked thousands of consumers out of millions of dollars.” Mr. Whitaker served on its board and once threatened a complaining customer, lending the weight of his former position as U.S. attorney for the Southern District of Iowa to the company’s scheme.

Finally, and fundamentally most damning, is Mr. Whitaker’s expressed hostility to Marbury v. Madison, a central case — the central case — in the American constitutional system. It established an indispensable principle: The courts decide what is and is not constitutional. Without Marbury, there would be no effective judicial check on the political branches, no matter how egregious their actions.

If the Senate were consulted, it is impossible to imagine Mr. Whitaker getting close to the attorney general’s office. He should not be there now.

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