Former Virginia Gov. Bob McDonnell, center, arrives at federal court with his children in 2014. (Steve Helber/AP)

The Supreme Court’s unanimous decision to vacate former Virginia governor Robert F. McDonnell’s public corruption conviction will significantly limit prosecutors’ ability to bring cases against politicians suspected of malfeasance and could spell trouble for the Justice Department in ongoing, high-profile cases, legal analysts said.

The ruling, which narrows what constitutes criminal corruption, will be an immediate boon for Sen. Robert Menendez (D-N.J.), who is awaiting an appeals court ruling in a corruption case, experts said. Lawyers for former New York State Assembly speaker Sheldon Silver (D) said the ruling also will be central to their client’s bid to overturn his conviction.

The facts of each case are different, and the high court’s decision in the McDonnell case provides no guarantees for anyone else. But lawyers said the Supreme Court has forced the Justice Department to reevaluate what it considers criminal when it comes to politicians interacting with those who give them money.

“It just raises the standard of prosecution to a very, very high level,” said Jessica Tillipman, an assistant dean at the George Washington University Law School who teaches an anti-corruption seminar. “I think it’s going to make it a lot easier for politicians to accept gifts and hospitality and payments in return for taking action.”

McDonnell, a Republican, and his wife, Maureen, were convicted in September 2014 of providing various sorts of help to Richmond businessman Jonnie R. Williams Sr. in exchange for $177,000 in loans, vacations and luxurious gifts. The former governor was sentenced to two years in prison, his wife to a year and a day.

McDonnell’s challenge to his conviction was largely based on the argument that he neither performed nor promised to perform any so-called “official acts” for Williams, and that jurors were instructed wrongly on what could constitute official acts.

Prosecutors had alleged McDonnell crossed the line by arranging meetings for Williams with state officials, allowing Williams to use the governor’s mansion for a luncheon on the day his product, a dietary supplement called Anatabloc, was being launched and personally touting the supplement to state human resources officials.

That was part of a broader effort, prosecutors alleged, to have Anatabloc studied by state researchers or included in the state’s health plan.

The Supreme Court ruled that jurors were instructed wrongly on the topic. An official act, the justices said, “must involve a formal exercise of governmental power, and must also be something specific.”

“Setting up a meeting, talking to another official, or organizing an event — without more — does not fit that definition of ‘official act,’” Chief Justice John G. Roberts Jr. wrote in the opinion. The justices remanded the case to a federal appeals court and left open the possibility that McDonnell could be re-tried if prosecutors had evidence to show that the former governor agreed to pressure state researchers to study Anatabloc or force the supplement’s inclusion in the state health plan.

Some lawyers argue that the Supreme Court’s decision was a necessary clarification of a vague law, and one that will prevent prosecutors from overreaching. McDonnell enjoyed widespread support from former state and federal officials who feared that upholding his conviction would criminalize routine interactions between politicians and those who make political or personal donations.

Jacob Frenkel, a white-collar criminal defense lawyer at the Dickinson Wright law firm, said the justices made the “right decision.”

“What these unanimous decisions are saying to prosecutors is, ‘Stay within a more narrow and reasonable definition of bribery and corruption. Don’t try to shoehorn conduct into the definition for purposes of leveraging a prosecution,’ ” Frenkel said.

Hank Asbill, a leading member of McDonnell’s defense team from the Jones Day law firm, said, “I always think it’s good when criminal statutes are drawn with precision or defined with precision, so people understand what’s in bounds and what’s out of bounds.”

U.S. Attorney Dana Boente, whose office prosecuted the case, said in a statement: “The U.S. Attorney’s Office is reviewing the Supreme Court’s decision in the McDonnell matter and does not have any further comment at this time.”

Randall Eliason, a former prosecutor who specialized in public corruption and government fraud, said an elected official could now demand $10,000 for arranging a meeting for a constituent and not run afoul of federal bribery law.

“I could make a lot of money if my policy was just, ‘Hey, you want me to tell my guy to take the meeting? You pay me,’ ” Eliason said.

Lawyers for Silver, the former New York State Assembly speaker, said in a statement the McDonnell decision “makes clear that the federal government has gone too far in prosecuting state officials for conduct that is part of the everyday functioning of those in elected office.”

Silver, 72, was convicted of taking millions of dollars in bribes and kickbacks in exchange for various favors, including directing state grants to a particular doctor who passed on leads to him about potential law clients. Silver was sentenced to 12 years in prison.

“The McDonnell decision will be central to Mr. Silver’s appeal,” his attorneys, Steven Molo and Joel Cohen, said.

Abbe Lowell, Menendez’s attorney, said the Supreme Court had confirmed “that the government has too much power in the area of prosecuting public officials, to sort of make up the law as it goes along.”

Lowell, who represented Williams’s former company in the McDonnell case, said prosecutors would now have to decide in the cases of Menendez and others “whether the allegations go to what the Supreme Court said was the public official’s authority to act, and to be doing more than what is the typical, expected constituent service that our system depends on.”

Menendez, who has yet to go to trial, is accused of trying to influence immigration-visa proceedings for a friend’s girlfriends; assisting the friend in a dispute with the Centers for Medicare and Medicaid Services; and advocating action to help the friend make money from a port security contract in the Dominican Republic. He is alleged to have done so in exchange for campaign contributions and posh getaways.

Menendez is appealing his case on the grounds that he is protected by the so-called “speech or debate clause,” which prohibits prosecutions of congressmen for legislative activity.