State prosecutors pursuing conduct that is the subject of a presidential pardon will have to navigate “double jeopardy” prohibitions in the Fifth Amendment and state law. State law prohibits prosecution of “two offenses based on the same act or criminal transaction,” regardless of whether they are federal or state offenses. There are ways around this; for example, the protection is not prompted until either a federal guilty plea is in place or the matter has proceeded to trial (with a jury empaneled, or the first witness sworn for a nonjury case) in the federal courts. But the situation is complex, to say the least, and deep experience with the criminal justice system is a must for the new attorney general.

The Legislature must also select someone who has the gravitas to work with federal prosecutors. To continue with the example of Mr. Cohen, the new attorney general should be someone who can make the case to the Department of Justice to focus its case against Mr. Cohen on just a few of the potential claims, such as Stormy Daniels-related campaign finance violations. That would leave other, separate offenses under New York law, such as bank fraud, for state prosecutors to pursue — another important exception to double jeopardy limits.

There is reason to believe the new attorney general will find a receptive audience. The guilty plea by Michael Flynn, Mr. Trump’s former national security adviser, covered such narrow terrain, omitting many other allegations against him, such as charges related to work he allegedly did for the Turkish government. That may have been to leave room for the states to prosecute his alleged uncharged offenses should Mr. Trump pardon him.

Of course, the legal hurdles would be lower still if the Legislature amended the New York double jeopardy statute to expressly exclude jeopardy in cases in which presidential pardons are issued. In that case, a pardon would be the functional equivalent of an appellate reversal of a case: It would allow the investigation and prosecution to start again from scratch in New York. Working with the Legislature on this will be another important task for the new attorney general, and it argues again for someone with profound analytical depth.

Other attorneys general across the country will also have important roles to play if Mr. Trump executes a pardon strategy, or succeeds in eviscerating the federal investigations through firings or otherwise. But because Mr. Trump and his circle have so many interests in New York, the state has emerged as a key player. For the moment, the office is in good hands under the acting head, Solicitor General Barbara Underwood. But with legislators preparing to install an interim replacement, they must select somebody with the reputation and the collegiality to help lead the multistate effort.

It will be tempting for the Legislature to move fast and appoint a placeholder, and let voters work things out at the polls in November. But a great deal could happen between today and the swearing-in of a newly elected attorney general in 2019. The crisis could strike at any moment, and if it does the people of this state — and the nation — will be looking to the New York attorney general for leadership. The Legislature must choose someone who is ready for that weighty role, right now.