"A sitting President cannot be indicted"

[The Velveteen Ocelot]

5. It's based on a DoJ and OLC policy formulated during the Watergate proceedings,

and based on legal analyses concluding that it would probably violate the Constitutional principle of separation of powers. But the law isn't settled at all.

The first OLC memo was issued on September 26, 1973, not long before the October 20 “Saturday Night Massacre,” in which Nixon directed officials of the Justice Department to fire special prosecutor Archibald Cox, and the top two Justice officials resigned, leaving the dirty work to then-Solicitor General Robert Bork.

That memo considers the Constitution’s text and finds no answer. It says, correctly, that there is no “airtight separation of powers, but rather … a system of checks and balances, or blending the three powers.” The Constitution provides very limited immunities for members of Congress and none for the president. The impeachment clause says that any official impeached can be tried—at least, but not clearly only, after removal. The debates during the framing and ratification of the Constitution suggest that the president is subject to laws like any citizen, but never discuss prosecution in office. During the trials of Aaron Burr, Chief Justice John Marshall had insisted that Thomas Jefferson was subject to subpoena—but also that as president he could refuse to attend court in person, and could withhold some evidence.

Left with no clear sources, the Justice Department lawyers asked what answer would best serve the nation. An indictment in office would besmirch the “symbolic head of the nation.” In addition, “only the president can receive and continuously discharge the popular mandate expressed quadrennially in the presidential election,” making an indictment or trial “politically and constitutionally a traumatic event.” Impeachment is the first line of defense against presidential misconduct, the author noted. “This would suggest strongly that … criminal proceedings against a President in office should not go beyond a point” that they would effectively remove a president, and thus become a short-cut for impeachment.

Much more discussion here: https://www.theatlantic.com/ideas/archive/2018/05/presidential-indictment/560957/