Former Virginia governor Robert F. McDonnell speaks outside the Supreme Court on April 27. (Andrew Harnik/Associated Press)

THE DEBATE over former Virginia governor Robert F. McDonnell’s favors-for-gifts-and-loans scandal has always turned on how to distinguish between official corruption on the one hand, and run-of-the-mill political sleaze on the other. On Monday a unanimous Supreme Court, in scrapping Mr. McDonnell’s conviction and sparing him from prison, at least for now, suggested that Mr. McDonnell’s conduct may have been of the latter variety.

Writing for the court, Chief Justice John G. Roberts Jr. offered a narrow and exceptionally permissive interpretation of what constitutes actual corruption. In doing so, he also created an aura of forgiveness on behalf of the sort of quotidian back-scratching, influence-peddling and favor-doing — much of it greased by cash — that repels and has soured so many Americans on politics as usual.

The court explicitly recognized that Mr. McDonnell’s actions were “distasteful,” or worse; they included accepting $175,000 worth of gifts and loans from a wealthy Virginia businessman, Jonnie R. Williams Sr. Mr. Williams sought and received the governor’s help in advancing the prospects of a tobacco-based diet supplement of no proven efficacy, while showering the pliable Mr. McDonnell and his even more willing wife with baubles, vacations and other tangible benefits that any layman would recognize as a big fat bribe.

Yet, Chief Justice Roberts wrote, the court’s concern was not “tawdry tales of Ferraris, Rolexes, and ball gowns” — a very partial list of the goods Mr. Williams showered upon the McDonnells — but whether the quid pro quo from the governor amounted to a specific and tangible “official act” that went beyond routine courtesies and services for a constituent.

The court didn’t answer that question, ruling only that the federal trial court in Mr. McDonnell’s 2014 trial had offered the jury an overly expansive definition of “official acts” that would trigger a bribery conviction. Prosecutors must now decide whether to retry Mr. McDonnell on the severely constricted legal standard for corruption the Supreme Court has now established.

Chief Justice Roberts insisted the government would still be able to make cases against corrupt officeholders. In fact, it will be much more difficult. In the McDonnell case, prosecutors showed that the former governor’s interventions on behalf of Mr. Williams often followed directly — in some cases, within minutes — after the businessman had greased his or his wife’s palm. Yet those interventions, which included strong suggestions that state officials help Mr. Williams by persuading state university researchers to test his firm’s diet supplement, may have been insufficiently overt or conclusive to meet the court’s narrow definition of an “official act.”

That will give comfort to other ethics-scorning politicians who face prosecutorial scrutiny now and in the future, including Sen. Robert Menendez (D-N.J.), who faces federal corruption charges.

Mr. McDonnell, who previously apologized to Virginians for the embarrassment his conduct in office caused them, now hails the court’s decision as vindication for his insistence that he broke no law. It may turn out that he’s right. If it does, however, the court’s ruling will mean that sticky-fingered public officials can skate right up to the line of outright bribery with far less trepidation and legal risk than good government and sound ethics demand.