DOJ has consistently taken the position that the president can’t be indicted and prosecuted. In 1973, amidst Watergate, the Office of Legal Counsel determined that impeachment is the sole remedy for presidential misdeeds—even if the inability to indict and prosecute the president results in “a complete hiatus in criminal liability.” 

While a 1988 OLC memo concluded that the president is not immune from being subpoenaed in a criminal case, it held that the president doesn’t have to testify in court if duties prohibit it. Most recently, in 2000, OLC reaffirmed the 1973 line: A sitting president shouldn’t be indicted. It would “unconstitutionally undermine the capacity of the executive branch to perform its constitutionally assigned functions.”

Take a step away from DOJ, though, and you’ll find two stronger memos proving 40 years’ confidence that the president is indictable. 

Other federal thoughts on indictability 

The Office of the Special Prosecutor investigating Nixon concluded in 1974 that Nixon could be, even had to be, indicted at the conclusion of its investigation. One of its strongest points, relevant here, is that a grand jury has a sworn, constitutional duty to “present” all offenses that come to the jurors’ knowledge. If a grand jury considering someone else’s indictment has evidence implicating the president, are they supposed to ignore it? Then there’s that thing about the “paramount importance of reaffirming the integrity of the law.” The authors also note, apropos of DOJ’s argument, that indictment and prosecution are less disruptive to government than impeachment.

Independent Counsel Ken Starr reached the same conclusion in May 1998. He leans on a few key, common sense arguments: No one’s above the law. The Constitution doesn’t grant immunity. A criminal proceeding wouldn’t supplant the impeachment mechanism. Finally, it’s nuts to think impeachment must proceed prosecution; that would put Congress functionally in charge of when an executive power can be exercised, violating the separation of powers.

The best writing on the subject comes not from DOJ or a counsel of any sort but from the judiciary—the U.S. Court of Appeals for the District of Columbia—in a case over what Nixon could be made to turn over to a grand jury. The 1973 opinion, which Nixon did not appeal, shreds the claim that the president isn’t indictable. 

Though the President is elected by nationwide ballot, and is often said to represent all the people, he does not embody the nation's sovereignty. He is not above the law's commands: "With all its defects, delays and inconveniences, men have discovered no technique for long preserving free government except that the Executive be under the law...." Sovereignty remains at all times with the people, and they do not forfeit through elections the right to have the law construed against and applied to every citizen.

The court decided the case en banc and per curiam. Which means (a) every D.C. Circuit judge was involved and (b) they were in such strong agreement they chose to make the decision from the court rather than naming individuals. The language is at points delightfully sharp. (Forgive the pun.)

Nor does the Impeachment Clause imply immunity from routine court process. While the President argues that the Clause means that impeachability precludes criminal prosecution of an incumbent, we see no need to explore this question except to note its irrelevance to the case before us.

In the time-honored tradition of snarky footnotes, they hid this gem at n. 50:

Because impeachment is available against all "civil Officers of the United States," not merely against the President, it is difficult to understand how any immunities peculiar to the President can emanate by implication from the fact of impeachability

The D.C. Circuit opinion was, of course, confined to a much narrower topic; it’s an appellate court, not the Supreme Court. But it’s a solid roadmap for any judges confronted with related issues.

DOJ’s the only one with the power to indict

Starr was an independent counsel; he operated outside of the executive branch. That position no longer exists. The 1978 Ethics in Government Act’s independent counsel provisions required reauthorization every five years. Congress let them lapse in 1999. The special counsel, unlike an independent counsel, is still subject to DOJ big-footing. Which means Rod Rosenstein can block Mueller even if Mueller were to advocate for an indictment. 

While a state attorney general could try to indict Trump, they’re unlikely to prevail. There’s a Supremacy Clause problem (which Starr acknowledged).

This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding.

Check out 1997’s Clinton v. Jones, decided by the Supreme Court:

Because the Supremacy Clause makes federal law "the supreme Law of the Land," any direct control by a state court over the President, who has principal responsibility to ensure that those laws are "faithfully executed," may implicate concerns that are quite different from the interbranch separation-of-powers questions addressed here.

The end

Even if DOJ were willing to indict, the question is less whether the president can be indicted than whether it’s a good idea to indict him. The first problem is the specter of constitutional crisis; the second, the threat of having Brett Kavanaugh cast a fifth vote to protect any and all occupants of the office of the president from indictment.

As friend Ian Millhiser said, though, don’t rule out the possibility that Trump will be indicted at 12:01 am on January 20, 2021.