When former special counsel Robert S. Mueller III declared that indicting a sitting president was “unconstitutional,” some legal experts were taken aback. The question has never been litigated and so never decided by the Supreme Court.

The reality, as Mueller said in the same breath — making his first public comments Wednesday on his Russia investigation — is that the prohibition on charging presidents with federal crimes is “long-standing department policy.” It was formulated in two opinions during the administrations of Richard Nixon and Bill Clinton by the Justice Department’s Office of Legal Counsel (OLC).

The opinions of this elite corps of lawyers are binding on the federal government — but are not the equivalent of Supreme Court rulings.

On question of obstruction, Mueller hewed to untested Justice Department opinion

For these lawyers are not “umpires,” as Chief Justice John G. Roberts Jr. once described the role of Supreme Court justices. They are the president’s lawyers, whose stated responsibilities include “facilitating the work of the Executive Branch and the objectives of the President, consistent with the law.”

The body of opinions they produce have been called “the law that presidents make.”

And it is no surprise that such law holds that presidents are not subject to criminal prosecution.

Sen. Elizabeth Warren (D-Mass.) said Friday in a blog post that as president, she would seek a reversal of that Justice Department policy. “When I’m elected, I will exercise my constitutional authority to appoint an Attorney General who shares my strong conviction that no one — not even a President — is above the law,” she said.

While the OLC is proud of its “independent and principled” judgment, a recent Cornell Law Review study of its work, as well as a review of its publicly available decisions, suggests that it has a hard time saying “no” to its chief client.

The yeses accumulate. And because the Supreme Court tends to stay away from separation-of-powers issues, an OLC opinion becomes precedent for the next OLC opinion on the same subject, and the next one and the one after that.

Before long, it’s all settled as far as the Justice Department is concerned.

That’s how White House claims of “executive privilege” have evolved. What began as a few historical examples of withheld information by a handful of presidents and a passing acknowledgment in the Supreme Court’s 1974 Watergate tapes case, U.S. v. Nixon, is now routinely invoked by presidents as fundamental law.

The OLC’s recent opinion supporting President Trump’s barring of House Judiciary Committee testimony by former White House counsel Donald McGahn is illustrative, citing “nearly five decades” of OLC precedent in support of Trump’s claim.

Similarly, the OLC cited prior opinions and “centuries of historical practice and precedents” in addition to its interpretation of the law, when it determined that Trump could name Matthew G. Whitaker acting attorney general, temporarily replacing Jeff Sessions, despite the fact that Whitaker had held no position confirmed by the Senate.

“Some people have described it as a one-way ratchet,” said Jonathan Shaub, who served in the OLC from 2014 through the summer of 2017. “A president does something. The next president comes along and says, ‘I can do this other thing that’s just a step further.’ ”

The OLC’s precedent, Shaub said, “sets the stage for future presidents, and most favor executive branch authority.”

Justice Department spokeswoman Kerri Kupec declined to comment.

Unlike a court, the OLC’s opinion-making process is neither neutral, adversarial nor transparent. Much of its work is kept secret. Some is informal, according to lawyers who have worked there, consisting of emails and purely oral communications.

All this is among the reasons executive branch power keeps expanding, often at the expense of Congress, particularly in areas involving war powers and national security.

On occasion, precedent doesn’t matter so much. The most recent example of that came when the OLC said the appointment of Trump’s son-in-law, Jared Kushner, as adviser to the president, did not run afoul of federal nepotism laws.

The decision, dated on Trump’s Inauguration Day, ran contrary to at least four earlier OLC opinions on similar questions.

The office said it had “reexamined the legislative materials” on nepotism. (The federal anti-nepotism law was enacted after Bobby Kennedy served as attorney general to his brother, President John F. Kennedy, and did not apply to Hillary Clinton, who served as health-care adviser to her husband, because a court ruled the first lady was a “de facto” White House official.)

The most famous reversal was the revocation by the OLC in 2004 of the “torture memos” authorized by the same office for the George W. Bush administration after the Sept. 11, 2001, terrorist attacks.

For a Cornell Law Review article, scholar Adoree Kim compared the OLC’s publicly released decisions advising executive branch agencies with those advising the White House and found a sharp divergence. While the office often felt free to say “no” to agency legal interpretations, it was far more obliging to the White House.

“The OLC is deeply deferential to the President and to presidential action,” she wrote, and “this deference is not unique to times of crisis. It is also not limited to the realm of foreign affairs, or to constitutional questions. Rather, the OLC demonstrates systematic partiality toward the President.” The office, she concluded, is indeed the “president’s law firm.”

But “as a practical matter,” said Walter Dellinger, who ran the office for three years under President Bill Clinton, “it is difficult to cut back on opinions expanding executive power,” though it can and has been done.

Deference to the president “is baked into OLC’s culture,” former OLC lawyer Erica Newland wrote recently in The Washington Post. It “proceeds from the assumption that the president is acting consistent with Article II of the Constitution and with his oath of office, both of which require that he ‘faithfully’ execute the laws.”

“But when I was at OLC, I saw again and again how the decision to trust the president failed the office’s attorneys, the Justice Department and the American people. The failure took different forms,” she wrote. “Sometimes, we just wouldn’t look that closely at the claims the president was making about the state of the world. When we did look closely, we could give only nudges.”

Newland left the office in November. “I decided that the responsibilities entailed in my oath were incompatible with the expectations of my job,” she wrote.