His potential criminal liability goes further, to actions before taking office. The Manhattan district attorney is by all appearances conducting a classic white-collar investigation into tax and bank fraud, and the New York attorney general is engaged in a civil investigation into similar allegations, which could quickly turn into a criminal inquiry.

These state matters may well reveal evidence warranting additional federal charges. Such potential financial crimes were not explored by the special counsel investigation and could reveal criminal evidence. Any evidence that was not produced to Congress in its inquiries, like internal State Department and White House communications, is another potential trove to which the new administration should have access.

The matters already set out by the special counsel and under investigation are not trivial; they should not raise concerns that Mr. Trump is being singled out for something that would not be investigated or prosecuted if committed by anyone else.

Because some of the activities in question predated his presidency, it would be untenable to permit Mr. Trump’s winning a federal election to immunize him from consequences for earlier crimes. We would not countenance that result if a former president was found to have committed a serious violent crime.

Sweeping under the rug Mr. Trump’s federal obstruction would be worse still. The precedent set for not deterring a president’s obstruction of a special counsel investigation would be too costly: It would make any future special counsel investigation toothless and set the presidency de facto above the law. For those who point to the pardon of Richard Nixon by Gerald Ford as precedent for simply looking forward, that is not analogous: Mr. Nixon paid a very heavy price by resigning from the presidency in disgrace for his conduct.

Mr. Trump may very well choose to pardon not just his family and friends before leaving office but also himself in order to avoid federal criminal liability. This historic turn of events would have no affect on his potential criminal exposure at the state level. If Mr. Trump bestows such pardons, states like New York should take up the mantle to see that the rule of law is upheld. And pardons would not preclude the new attorney general challenging a self-pardon or the state calling the pardoned friends and family before the grand jury to advance its investigation of Mr. Trump after he leaves office (where, if they lied, they would still risk charges of perjury and obstruction).

In short, being president should mean you are more accountable, not less, to the rule of law.

Andrew Weissmann, a senior prosecutor in Robert Mueller’s special counsel investigation, is a senior fellow at the New York University School of Law and the author of “Where Law Ends: Inside the Mueller Investigation.”