If confirmed by the Senate to a Supreme Court seat, Judge Brett Kavanaugh immediately could be a crucial vote to decide whether the constitution bars any state’s efforts to prosecute Trump campaign officials and aides for state crimes if President Trump has already pardoned them for federal crimes.

Those who believe that the president will pardon Trump campaign officials who are indicted or convicted in federal court have long viewed state-level prosecutions as a back-up plan to bring them to justice. Specifically, the same actions that would support federal criminal charges brought by Special Counsel Robert Mueller also would support a prosecution for violation of state criminal laws.

But a little-noticed case currently pending before the U.S. Supreme Court could block this exact scenario, not only for those already found guilty or who pleaded guilty on federal charges, such as Paul Manafort or Michael Cohen, but for any Trump or White House aide whom Trump might preemptively pardon before Mueller files federal criminal charges.

The obscure case is Gamble v. United States, and the U.S. Supreme Court will decide it this term. Terrance Gamble, a convicted felon, was prosecuted by the state of Alabama for illegal possession of a firearm and sentenced to prison. Subsequently, the federal government also indicted Gamble under federal law for the same act. Gamble moved to dismiss the federal indictment.

Gamble has thus far unsuccessfully argued in two federal courts that the “double jeopardy” clause of the Fifth Amendment barred the federal prosecution. Both the district court and the Eleventh Circuit Court of Appeals rejected Gamble’s position and, the latter stated: “The Supreme Court has determined that prosecution in federal and state court for the same conduct does not violate the double jeopardy clause because state and federal governments are separate sovereigns.”

Gamble sought Supreme Court review. And, if he wins, the federal prosecution against him will be barred because the court will have ruled, for the first time, that the double jeopardy clause in the Bill of Rights forbids both the federal and state government from prosecuting a defendant based upon the same conduct.

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Surprisingly the Supreme Court in late June agreed to accept Gamble’s appeal, even though no lower court had reached a contrary outcome. The first Supreme Court opinion recognizing the right of both the state and federal government to prosecute a person for the same act was decided in 1847 in Fox v. Ohio. Plus, the modern-era precedent in U.S. v. Lanza (1922) reaffirmed the right of both federal and state jurisdictions to prosecute a defendant for the same acts if both federal and state laws were violated.

At least four justices must agree to accept a case for review and doing so does not necessarily mean that the four voting justices intend to reverse the lower court. But when, as is the case in Gamble, there are no conflicting rulings in the lower courts, it’s uncommon to simply take a case to re-reaffirm those courts. In fact, it is fair to speculate that at least four justices are willing to consider reversing these long-standing precedents.

Whether a fifth justice would join them would depend upon who is on the court when the oral argument — not yet scheduled — is heard.

Gamble’s efforts to overturn a 170-year-old constitutional doctrine have an ally in a leading Republican senator.

Sen. Orrin Hatch, R-Utah, a staunch defender of Kavanaugh during the Senate Judiciary Committee hearings, filed a “friend of the court” brief on September 11 — while the Kavanaugh nomination was pending — in support of Gamble’s petition to overrule the dual sovereignty doctrine.

In his amicus brief, lawyers for Senator Hatch urged that the “court should overrule its prior decisions upholding the due sovereignty doctrine.” The brief then contended that the expanding scope of federal criminal law in modern times has so overlapped with traditional state criminal statutes that the historical reasons for allowing a separate enforcement no longer exists. While he also argued that existing law permitting dual prosecutions makes it more difficult for lawmakers to set punishments for crimes, as the defendant could be convicted and punished twice, no other lawmaker joined in the senator’s brief.

So, if confirmed, then-Justice Kavanaugh would be able to vote whether to overturn settled Supreme Court precedent dating back to the 1840s that allows both federal and state governments to prosecute a person for the same actions. If these precedents were overruled, a presidential pardon would, in effect, shield guilty parties from criminal liability on both the federal and state level.

Critics of Judge Kavanaugh’s appointment, citing to his views that a sitting president cannot even be investigated much less indicted, have already warned that he could undermine Mueller’s investigation in deciding these issues if they came before the Supreme Court in a hypothetical future case.

But there is nothing hypothetical about the impact if the court overrules settled law in Gamble.

As widely-noted, the issuance of a pardon by President Trump only immunizes a person from prosecution for federal crimes. For example in the Manafort case, his conviction under federal law for bank fraud by a Virginia federal jury would be extinguished if President Trump pardoned him. But, under existing constitutional principles, the pardon would not bar Manafort from being prosecuted under state law in Virginia for the same underlying act.

A ruling in favor of Gamble would fundamentally change that outcome.

Moreover, the issuance of pardons could also be extended to Trump aides or family members not yet charged by Mueller’s team as a preemptive effort to halt any prosecution. If Gamble prevails in the Supreme Court, anyone pardoned would be immunized from state court prosecutions based on those same acts.

The ninth Supreme Court justice could be the deciding vote whether to overrule existing constitutional principles and that could well be Brett Kavanaugh.