The Double Jeopardy Clause of the Constitution says someone can't be prosecuted twice for the same crime.
But, in fact, for 160 years, the Supreme Court has said that separate sovereigns — state and federal governments — can do just that, because each sovereign government has separate laws and interests.
Now, the Supreme Court could be ready to reverse that long-standing rule — and that could have consequences for the ongoing investigation of President Trump and his associates led by special counsel Robert Mueller and their ties to Russia.
Indeed, a decision barring such dual prosecutions could allow some of those already convicted in the Mueller probe to get off scot-free if President Trump were to pardon them, something that he has openly flirted with doing.
When do double prosecutions occur?
Double prosecutions most often come in high-profile cases, frequently civil-rights cases, where there is a sense that justice has not been done.
In 1992, for instance, riots broke out in Los Angeles after a mostly white jury in the nearby suburb of Simi Valley acquitted four police officers in the videotaped beating of a black man named Rodney King.
The day after the state-court verdict, Attorney General William Barr declared that "nothing in the state process is binding on us federally," and, indeed, several months later, federal prosecutors filed charges against the same police officers for violating King's civil rights.
A more racially mixed federal jury in Los Angeles subsequently convicted two of the officers.
What's the case before the Supreme Court?
The case the court is hearing Thursday is far more mundane than the King police brutality case. In 2015, seven years after Terance Gamble was convicted of robbery in Alabama, he was pulled over by police for a traffic violation.
When the cops found a handgun and two bags of marijuana in the car, the state charged him with violating an Alabama law barring convicted felons from possessing a firearm.
Gamble pleaded guilty to the state charges and was sentenced to one year in prison, with the rest of his sentence suspended. His subsequent conviction for violating a nearly identical federal statute, however, added three more years of prison time.
Gamble appealed the second conviction, contending it violated the U.S. Constitution's ban on double prosecution for the same crime. But the lower courts ruled that, under the Supreme Court's established precedents, the state and federal governments, as separate sovereigns, are allowed to bring such successive prosecutions.
Now, Gamble's case is before the Supreme Court, where two justices at opposite ends of the ideological spectrum — Ruth Bader Ginsburg and Clarence Thomas — have suggested it may be time to revisit the separate-sovereigns doctrine.
A get-out-of-jail-free card for Trump associates?
The case has attracted extra attention because of President Trump's comments that he could possibly pardon his onetime campaign chairman Paul Manafort and other Trump associates who have been — or could be — convicted in prosecutions brought by Mueller.
Presidential pardons, it should be noted, only apply to federal crimes. So, under current law, a state like New York, for example, could go after Manafort for the same crimes, but using state laws.
If the Supreme Court were to bar dual prosecutions, "there is a concern that a president of the United States could pardon an individual for all federal offenses" and it would effectively be a "pardon for everything," said George Washington University Law Professor Stephen Saltzburg, who served as deputy assistant attorney general in the Reagan and Bush administrations.
These concerns, he noted, extend from the president down to governors or local prosecutors. They could "bestow great gifts upon friends or family by rushing to prosecute them for certain crimes," Saltzburg pointed out, by obtaining minimal punishments.
That would be "cutting off" the ability of the federal government to bring prosecutions, for the same conduct, particularly in corruption cases.
But Columbia Law School professor Daniel Richman, who previously headed the appeals section at the U.S. Attorney's office in Manhattan, is not very worried about the Manafort case, or others like it.
Manafort, he said, is "sort of a one-man offense generator."
Put another way: If a prosecutor can't find different state crimes to charge someone like Manafort with, they should be in a different job.
Possible state charges could include money laundering or real-estate fraud, according to Richman, who observes that every money-laundering transaction is a separate crime, as is every false statement on a state form or state tax filing.
In a similar vein, Erwin Chemerinsky, dean of University of California, Berkeley School of Law, notes that the practical impact of a rule prohibiting separate sovereign prosecutions could be relatively minor, since federal and state crimes usually require proof of different elements.
Thus, for example, an acquittal on murder charges in state court may be followed by a federal prosecution for violating the victim's civil rights. And while the actual criminal act — killing — is the same, federal prosecutors may have to prove, for example, racial, ethnic or gender animus.
"What will be argued over and again is when are the elements sufficiently similar" that the second prosecution violates the Constitution's ban on double jeopardy, Chemerinsky said.
A low-level criminal with a high-stakes case
Terance Gamble, the defendant at the center of Thursday's Supreme Court case, is the relatively rare defendant prosecuted by the state and federal governments for exactly the same crime, but who is not involved in major criminal activity.
He was prosecuted on federal charges because the U.S. attorney in Alabama and the Obama administration were flagging felons with a history of domestic violence and violent gun use. Gamble had a history of terrorizing domestic partners with a gun.
Gamble's lawyers contend that, nonetheless, the double-jeopardy provision of the Constitution bars double prosecution and punishment for the same crime, and that the nation's founders never contemplated an exception that would allow the state and federal governments each a separate bite at the prosecution apple.
They argue it is time to overrule the separate sovereigns doctrine, because it was wrong to begin with and, because "the dramatic federalization of criminal law over the last 60 years" has "rendered almost laughable" the assumption that federal and state jurisdictions will only rarely overlap.
The federal government counters that the separate-sovereigns doctrine dates back nearly two centuries and that it serves the distinct and separate interests of the state and federal governments.
In Washington, there are always rumors of hidden motives in a case like this. And, indeed, some in liberal circles have speculated that one reason the Trump administration was in such a hurry to get Brett Kavanaugh confirmed to the Supreme Court was his demonstrated affinity for presidential power.
But, ironically, the Trump administration is urging the Supreme Court to preserve the status quo by upholding the separate-sovereigns doctrine, as are 36 states, including states that lean "red" and "blue" politically.
The Supreme Court, though, has already signaled how seriously it is considering reversing its long-standing precedents on double jeopardy — it expanded the time for Thursday's oral arguments by 20 minutes, showing just how seriously it is taking the case.