The Supreme Court unanimously voted to overturn the public-corruption conviction of former Virginia governor Robert F. McDonnell. Here’s what you need to know about the decision. (Monica Akhtar/The Washington Post)

The Supreme Court unanimously overturned former Virginia governor Robert F. McDonnell’s public-corruption conviction Monday and imposed higher standards for federal prosecutors who charge public officials with wrongdoing.

Chief Justice John G. Roberts Jr. described the former governor’s actions as “tawdry” but agreed that instructions to the jury in his case about what constitutes “official acts” were so broad, they could cover almost any action a public official takes.

McDonnell’s promising political career was derailed by his entanglement with a businessman who showered the governor and his family with luxury gifts and financial benefits. McDonnell and his wife, Maureen, were indicted and convicted after he left office in January 2014.

Legal experts said the new standards would make it difficult for federal prosecutors to retry McDonnell, but Roberts said the Supreme Court was expressing no view on that.

“If the court below determines that there is sufficient evidence for a jury to convict Governor McDonnell of committing or agreeing to commit an ‘official act,’ his case may be set for a new trial,” Roberts wrote. “If the court instead determines that the evidence is insufficient, the charges against him must be dismissed.”

The chief justice seemed concerned that the public might take the view that the court was endorsing politics as usual.

“There is no doubt that this case is distasteful; it may be worse than that. But our concern is not with tawdry tales of Ferraris, Rolexes, and ball gowns,” Roberts wrote. “It is instead with the broader legal implications of the Government’s boundless interpretation of the federal bribery statute.”

McDonnell expressed gratitude for the justices’ decision. “Today, a unanimous United States Supreme Court vacated my convictions, and it is a day in which my family and I rejoice and give thanks,” he said in a statement. “From the outset, I strongly asserted my innocence. . . . I have not, and would not, betray the sacred trust the people of Virginia bestowed upon me during 22 years in elected office. It is my hope that this matter will soon be over and that my family and I can begin to rebuild our lives.”

Maureen McDonnell’s conviction is still on appeal at a lower court. But her lawyer William A. Burck said the Supreme Court decision means that her conviction should “immediately be tossed out as well, which we are confident the prosecutors must agree with. Mrs. McDonnell, like her husband, was wrongfully convicted.”

The decision technically leaves federal prosecutors a window to press their case against Robert McDonnell, though they were mum about their intentions Monday.

Experts said they would face an uphill battle should they decide to do so. The Supreme Court remanded the case to the U.S. Court of Appeals for the 4th Circuit so that it might determine whether there is sufficient evidence that McDonnell performed or agreed to perform an “official act” under the clarified definition.

The Post’s Robert Barnes explains the big decisions from this Supreme Court session and how the absence of a justice will continue to affect the court. (Gillian Brockell/The Washington Post)

Jessica Tillipman, an assistant dean at George Washington University Law School who teaches an anti-corruption seminar, said “it’s feasible” that government lawyers could reframe the McDonnell prosecution in light of the new instructions.

“But it’s going be tough, and it also raises a lot of questions about the scope in future prosecutions, and as guidance to public officials going forward how they can act within the bounds of the law.”

Government watchdog groups were disheartened by the ruling.

Tara Malloy, deputy executive director for the Campaign Legal Center, said that it “makes it even more difficult to protect our democracy from attempts by officeholders to peddle political access and influence to the highest bidder.”

Democracy 21 President Fred Wertheimer said that the ruling “belies reality. If you show the facts in the case to any citizen, the citizen will conclude that the public official has sold his office for personal, financial gain.”

The McDonnell case stemmed from more than $175,000 in loans and gifts — a Rolex watch, vacations, partial payments for a daughter’s wedding reception among them — that the governor and his family received from Richmond businessman Jonnie R. Williams Sr. Williams, then the chief executive of Star Scientific, wanted state universities to perform important clinical tests on a dietary supplement the company had developed.

The gifts were not barred by Virginia law, and the tests were not conducted. But federal prosecutors said Williams’s generosity was part of an illegal quid pro quo arrangement. McDonnell’s part of the deal, they said, came in the form of meetings arranged to connect Williams with state officials, a luncheon Williams was allowed to throw at the governor’s mansion to help launch the product and a guest list he was allowed to shape at a mansion reception meant for health-care leaders.

McDonnell and his lawyers said he never took any official state action to benefit Williams. He amassed an outpouring of support from officials in both parties who told the court that upholding his conviction would criminalize the routine favors politicians do for benefactors and give federal prosecutors too much power.

In writing the court’s unanimous ruling, Roberts agreed that the government and the lower courts took too broad a view of when a politician’s actions can be considered nefarious.

“Conscientious public officials arrange meetings for constituents, contact other officials on their behalf, and include them in events all the time,” the chief justice wrote. “The basic compact underlying representative government assumes that public officials will hear from their constituents and act appropriately on their concerns — whether it is the union official worried about a plant closing or the homeowners who wonder why it took five days to restore power to their neighborhood after a storm.”

He said the government’s position “could cast a pall of potential prosecution over these relationships” if the union or group made some show of gratitude.

The opinion showed that the justices struggled to identify exactly what an “official act” encompasses, but they were clear about what it is not: “Setting up a meeting, calling another public official, or hosting an event does not, standing alone, qualify as an ‘official act.’ ”

Roberts said that for prosecutors to prevail, they must identify a “question, matter, cause, suit, proceeding or controversy” that “may at any time be pending” or “may by law be brought” before a public official.

Second, the government must prove that the public official made a decision or took an action on those matters, which Roberts said must be “of the same stripe as a lawsuit before a court, a determination before an agency or a hearing before a committee.”

Roberts indicated that the meetings and introductions a public official makes are not themselves official acts but could be seen as evidence that the officeholder was up to no good.

“A jury could conclude, for example, that the official was attempting to pressure or advise another official on a pending matter,” Roberts wrote. “And if the official agreed to exert that pressure or give that advice in exchange for a thing of value, that would be illegal.”

Hank Asbill, one of McDonnell’s attorneys, said, “I think we’ve proved the governor didn’t get a fair trial because he was prosecuted on flawed legal instructions.”

Roberts took pains not to excuse McDonnell’s conduct. Even though McDonnell and his supporters said his conviction made routine acts performed by politicians illegal, Roberts disagreed.

“None of this, of course, is to suggest that the facts of this case typify normal political interaction between public officials and their constituents,” he wrote. “Far from it. But the government’s legal interpretation is not confined to cases involving extravagant gifts or large sums of money, and we cannot construe a criminal statute on the assumption that the government will ‘use it responsibly.’ ”

Matt Zapotosky contributed to this report.