President Trump tweeted Monday morning that “numerous legal scholars” have said that he has “the absolute right” to pardon himself.

But in the past, the official position of the executive branch was that a president could not self-pardon.

The Constitution says that the president “shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.”

It’s true that this text doesn’t explicitly rule out the possibility of a self-pardon. (And it’s also an unanswered legal question as to whether a president can be indicted while in office.)

But a self-pardon would run afoul of a bedrock legal principle in the United States, according to a 1974 memo written by the Office of Legal Counsel under President Nixon. (The Supreme Court has never actually ruled on the issue.)

Read More: President Trump Says He Can Pardon Himself. Most Voters Disagree

“Under the fundamental rule that no one may be a judge in his own case, the President cannot pardon himself,” the memo declared days before Nixon resigned amid the Watergate scandal.

The document does say there could be a loophole.

“If under the 25th Amendment the President declared that he was temporarily unable to perform the duties of the office, the Vice President would become Acting President and as such could pardon the President,” it says. “Thereafter the President could either resign or resume the duties of his office.”

Outside of the legal questions, there would certainly be political complications if Trump tried to pardon himself, and the blowback could result in the Republican-controlled House impeaching him to remove him from office.

Over the weekend, Trump’s lawyer Rudy Giuliani also argued that Trump would have the power to pardon himself, but said he probably wouldn’t because of the “unthinkable” political ramifications. “It could lead to impeachment,” Giuliani said on NBC’s Meet the Press of Trump pardoning himself. “If he terminated an investigation of himself it could lead to all kinds of things.”

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Write to Tessa Berenson at tessa.berenson@time.com.