Tom DeLay. John Edwards. Rick Perry. Robert Menendez.

Like these politicians, Robert McDonnell is not your average defendant in the American criminal justice system. Nor is the former governor of Virginia the easiest man to root for, after receiving more than $177,000 in luxury items, vacations, loans, and other largess from a Virginia businessman that led to a conviction on federal corruption charges last year.

But he does make for an intriguing portrait of the state of political corruption in the United States.

This week, Mr. McDonnell’s legal team told the US Supreme Court that he, like those other men, is a victim. He says he has run afoul of a “criminalization of politics” – a growing trend wherein overzealous (and perhaps ambitious) prosecutors use vague or overbroad corruption laws to levy charges for what could be construed as everyday political activity.

The prosecutors, meanwhile, suggest that allowing McDonnell to walk free would lead to unchecked political corruption. At a time when the political system is increasingly influenced by megadonors, the stakes are growing, they add.

That makes the Supreme Court case, to be decided later this spring, a crucial test: Where, exactly, does the nation’s highest court want to set the bar for what is corruption and what is not?

Comments from the court and experts suggest that many think the pendulum has swung too far toward aggressive prosecutors and needs to be recalibrated. The court appears poised to do that. How far it will go is the question.

“People sort of accept that our political system as it’s currently set up anticipates a certain amount of low-grade corruption,” writes Matthew Kaiser, an expert on white-collar criminal defense, in an e-mail to the Monitor. For example, “every politician spends a lot of time asking for money and doing things in return for that money.”

“That situation may be troubling,” he adds, “but when it ought to be a crime is very hard to identify in a principled way.”

Recent cases suggest that prosecutors might have been overreaching. The prosecutions of Mr. Delay, a former House majority leader; Mr. Perry, a former governor of Texas; and Mr. Edwards, a former Democratic senator and vice presidential candidate; all were either unsuccessful or later overturned.

McDonnell is hoping the court will decide in a similar fashion. While he is not disputing that he received the gifts, he is disputing whether his largely passive responses – including arranging meetings with state officials for the businessman, Jonnie R. Williams Sr., and allowing him to throw a luncheon at the governor’s mansion to launch a product – qualified as corruption under federal law. In a quid pro quo (“this for that”) sense, there was nothing illegal about what he offered Mr. Williams, McDonnell argues.

The concern is that, if the court rules for McDonnell, his behavior will become more commonplace.

“The implication would be that that subtle corruption is going to be more difficult to prove,” says Randall Eliason, a professor at George Washington University Law School and an expert on public corruption. “Anything that makes proving corruption cases more difficult, the corollary of that is it makes it easier for the corruption to take place.”

On the flip side, a decision upholding McDonnell’s conviction – which could also result from a 4-to-4 tie – could act as a stimulant for prosecutors.

“There’s this concern about the criminalization of politics,” says Richard Hasen, a professor at the University of California, Irvine School of Law. “If the [federal anticorruption] laws are very vague, it gives room for overzealous prosecutors to bring these cases to make a name for themselves.”

Five White House counsels have signed an amicus brief in support of McDonnell.

For the White House lawyers “to agree on something as sensitive as this, and to be willing to put their names on something that says this cannot be prosecuted conduct, I think is extraordinary,” said Chief Justice John Roberts during oral arguments Wednesday.

But a broad ruling for McDonnell could leave the interpretations of federal corruption laws ambiguous.

“My problem is … how do we write those [laws] so that they do catch people who are doing the dishonest thing without, as I’ve said five times, allowing the government the freedom to go and do these ridiculous cases,” said Justice Stephen Breyer.

For this reason, experts are anticipating a ruling in favor of McDonnell that clarifies anticorruption laws without giving room for “pay to play” politics to flourish.

“The question is not if [McDonnell] wins, but how he might win … [and] whether the court says anything broadly about whether it’s permissible under this law to sell access for money,” says Professor Hasen. “I’m guessing the court will find way to side with McDonnell without going that far.”

The ultimate outcome could narrow federal anticorruption laws.

A narrower definition of corruption is “probably going to let some bad guys get away,” said William Haun, a private attorney representing business and political interests that filed an amicus brief supporting McDonnell, in a conference call Wednesday. “The question is: Should that be acceptable? And I would say that is acceptable.”

“The idea that people would be self-interested and self-aggrandizing with the political process is not exactly a new insight,” he said, “and the answer isn’t to send everyone to prison for this.”

Justice Breyer seemed to agree – to a point.

Any revised definitions of corruption “will leave some dishonest conduct unprosecuted. They won't be perfect,” he said.

But he added, “They will put some politicians at risk,” too.

An immediate beneficiary could be Senator Menendez (D) of New Jersey, who has been indicted on federal corruption charges. Prosecutors allege that Menendez accepted nearly $1 million in contributions and gifts from Saloman Melgen, a Florida eye doctor, in exchange for political favors. Menendez has said the two men have been friends for decades and were only helping each other.

Beyond that, the McDonnell decision could represent a “sea change,” writes Mr. Kaiser, the white-collar criminal defense expert at Kaiser, LeGrand & Dillon in Washington.

The “kind of deference prosecutors get from federal judges” in corruption cases could shrink dramatically.

[Editor's note: The original incorrectly identified Senator Menendez's state and what he is alleged to have received from Mr. Melgen.]