Dangling the prospect of presidential pardons is President Donald Trump's favorite new thing. He has publicly mused about pardons for everyone from Martha Stewart to the late Muhammad Ali (Ali’s conviction was unanimously overturned by the Supreme Court so he doesn’t need one). He has also tweeted that he has the right to pardon himself.

As has been stated by numerous legal scholars, I have the absolute right to PARDON myself, but why would I do that when I have done nothing wrong? In the meantime, the never ending Witch Hunt, led by 13 very Angry and Conflicted Democrats (& others) continues into the mid-terms! — Donald J. Trump (@realDonaldTrump) June 4, 2018

And yet, despite the cascade of comments from Trump and television interviews by his lawyer Rudy Giuliani about possible beneficiaries of presidential pardons or clemency, the potential targets of special counsel Robert Mueller should not be counting on a presidential pardon as a Get Out of Jail Free card. Trump’s pardons, if the president actually ever grants them, could signal the beginning, not the end, of criminal prosecution.

While a presidential pardon is a potent remedy, the pardon’s scope is in fact limited. Trump can only pardon federal crimes. But how can a state prosecutor, who had never launched an investigation, possibly have the evidence to prosecute suspects under state fraud, bribery, conspiracy or other laws? The answer lies in a largely unnoticed, post-Watergate change made by the U.S. Supreme Court in the federal rules governing secret grand jury information.

While a presidential pardon is a potent remedy, the pardon’s scope is in fact limited. Trump can only pardon federal crimes.

As outlined in the U.S. Attorney’s Manual, in 1985 the Supreme Court adopted a new provision in the Federal Rules of Criminal Procedure to remove “an unreasonable barrier to the effective enforcement of our two-tiered system of criminal laws” by allowing federal prosecutors to give their otherwise-secret grand jury investigative materials to state prosecutors looking to charge federal targets under state criminal statutes.

Mueller would have to obtain permission from his Department of Justice superior, presumably Deputy Attorney General Rod Rosenstein, for the transfer. How difficult this permission would be to obtain depends a bit on whom Trump had pardoned — granting clemency to his indicted aides or family members would likely lead to an enormous outrage and therefore provide political cover for the DOJ to turn over evidence to state prosecutors. If Mueller obtains permission, the information handed over to state prosecutors would be a treasure trove of indictment-ready evidence, giftwrapped by the special counsel’s office.

Suppose, for example, that Paul Manafort is tried and convicted in federal court. Trump could certainly issue a pardon and overturn the conviction, as he did with convicted former Arizona sheriff Joe Arpaio. But again, Manafort would still be potentially subject to state prosecution.

While viewers of television court dramas might think that the Constitution’s double jeopardy clause in the Fifth Amendment would protect Manafort, they would be mistaken.

While viewers of television court dramas might think that the Constitution’s double jeopardy clause in the Fifth Amendment would protect Manafort, they would be mistaken. Almost 100 years ago, the U.S. Supreme Court ruled in U.S. v. Lanza (1922) that the double jeopardy clause does not bar a person from being prosecuted under both federal and state law under the dual sovereignty doctrine.

"It follows that an act denounced as a crime by both national and state sovereignties is an offense against the peace and dignity of both and may be punished by each,” the court wrote in its decision.

Dual sovereignty is not an ancient or outmoded doctrine. A little more than a decade ago, NFL quarterback Michael Vick was prosecuted and pled guilty in federal court and subsequently in Virginia state court to charges arising out of the same dog-fighting operation. If a counterpart state law applies to the same conduct — e.g. bank fraud, conspiracy, witness tampering, obstruction of the administration of law enforcement — prosecution can proceed.

Some states, including New York, have enacted laws that bar a second prosecution based upon the same facts, if the defendant previously stood trial and all elements of the state and federal law are the same. But the elements of the criminal violation may not be the same, even if based on the same facts. A defendant tried for federal income tax evasion and acquitted could still be tried on the same facts for state income tax evasion, as the elements of the crimes differ. (And even if the crimes are the same, New York does not afford constitutional protection, but only a statute that can be amended or repealed by its legislature.)

All of this is to say that there are ways for prosecutors to get around a presidential pardon in the pursuit of justice. The targets of Mueller’s probe should remain highly skeptical, especially big names. Indeed, a plea deal in hand may yet prove to be the wisest course.

Michael M. Conway was counsel to the U.S. House Judiciary Committee in the impeachment inquiry of President Richard M. Nixon in 1974. Conway is a graduate of Yale Law School, a fellow of the American College of Trial Lawyers and a retired partner of Foley & Lardner LLP in Chicago.