Scholars have debated the question of whether the US Constitution allows a sitting president to be indicted and tried while in office — and whether it would be prudent to do so.
Two decades ago, Independent Counsel Ken Starr confronted these questions while investigating President Bill Clinton and answered yes to the first, but no to the second.
We don't know what Special Counsel Robert Mueller thinks of these questions — with respect to his investigation of the relationship between members of President Donald Trump's inner circle and Russian interference in the 2016 election — but it doesn't matter.
A change in law since 1999 has taken the decision out of his hands.
Mueller won't indict Trump. Here's what you need to know to understand what may happen.
The title "independent counsel" effectively meant that for the Clinton investigation, Starr was the Attorney-General, with the ultimate power to decide whom to prosecute and on what basis.
In response to concerns that independent counsels — not just Starr — were too powerful, Congress let the independent counsel statute expire in 1999.
In its place, the Department of Justice published regulations that enabled the appointment of a "special counsel" with narrower authority. Mueller functions more like a US Attorney.
He's confined in two ways: Firstly, Mueller must abide by all Department of Justice "rules, regulations, procedures, practices and policies". Secondly, any investigative or prosecutorial decision Mueller makes may be reviewed and disapproved by the Attorney-General. Deputy Attorney-General Rod Rosenstein is acting attorney-general for Mueller's purposes.
In 1973, the Justice Department's Office of Legal Counsel — essentially the government's lawyers' lawyers — determined that the president is constitutionally immune from indictment and criminal prosecution while in office.
With his broad authority, Starr felt unbound by that opinion, and ordered his own from a conservative legal scholar who determined that Starr could, in fact, indict Clinton. In 2000, OLC reaffirmed its earlier opinion.
Even if Mueller does conclude that Trump has committed a crime, and that it would be both constitutional and prudent to indict him, and even if Rosenstein agrees in all regards — all of which are big "ifs" — Rosenstein almost certainly would not allow such an indictment.
Starr issued a 445-page report cataloging Clinton's sexual relationship with White House intern Monica Lewinsky, outlining how the facts could give rise to impeachment.
This report was mandated by the independent counsel statute, which required Starr to advise Congress of "any substantial and credible information" that "may constitute grounds for an impeachment".
Clinton was impeached by the House but acquitted by the Senate. But Mueller can't do this, at least not directly. The special counsel regulations neither mandate nor allow Mueller to transmit a report to Congress.
He must provide a "confidential report" to the Attorney-General — Rosenstein, in Jeff Sessions' stead — explaining his "prosecution or declination decisions". The regs don't say what format that report must take or what happens next.
If Mueller believes he has information that could warrant impeachment, he could weave it into a narrative like the Starr Report. But even if Rosenstein wanted to make the report public, he would be limited by Federal Rule of Criminal Procedure 6(e), which imposes strict limits on the disclosure of grand jury materials.
This rule, which has the force of law, is intended to preserve the integrity of grand jury investigations and encourage witnesses to testify fully and frankly. Rosenstein could, if he chose, issue a redacted report that conveys the gist of Mueller's findings.
It's possible that by then Trump will have replaced Rosenstein with a more compliant successor who wouldn't affirmatively disclose the report. Or Trump could replace Sessions with a new Attorney-General who isn't recused from supervising Mueller.
Congress could subpoena the report in its entirety pursuant to recognised exceptions to grand jury secrecy. Congress could procure all investigative files that relate to the President. It could then make the material public if it so chose.
Only committee chairpersons can issue subpoenas. With the House and Senate under GOP control, Republicans could just decline to do so.
The 2018 midterm elections — and the potential for one or both houses to change hands — may be crucial to determining whether and when Mueller's Trump-related work sees the light of day.
Mueller is an investigator not a politician. His likely goals are, and should be, to dig up facts and bring them to light as warranted.
He might be able to name Trump as an unindicted co-conspirator, an unusual step that prosecutors sometimes deploy when it is impossible or impracticable to indict a specific person who participated in a criminal conspiracy.
Justice Department rules impose significant limits on this practice and it is uncertain whether Rosenstein would allow Mueller to do so.
Mueller could trigger a reporting requirement in the special counsel regulations that requires the Attorney-General to inform "the Chairman and Ranking Minority Member of the Judiciary Committees of each House of Congress" — both parties — at the end of the investigation, of any instance in which the Attorney-General vetoed a proposed action. But this would be of only limited scope.
What all these scenarios have in common is that Congress, not the courts, will be the ultimate arbiter of whether Trump faces any direct consequences. Depending on the state of play come November, voters may be the ones who determine whether Congress decides even to consider that question.
Elections have consequences, and one consequence of the midterms may be whether the American public gets to learn what Mueller has learned about Trump.
• Eric Columbus served as senior counsel to the deputy attorney-general in the Department of Justice and as special counsel to the general counsel in the Department of Homeland Security during the Obama Administration.