Such favors were at the heart of the case against Mr. McDonnell. During a period of personal financial turmoil, he received a Rolex, loans, trips, clothing and other benefits from Jonnie R. Williams Sr., a wealthy businessman who was seeking the governor’s help in securing state testing of a dietary supplement.

While the governor arranged meetings, made recommendations and appeared with Mr. Williams, the court ruled that he may never have committed an “official act” on Mr. Williams’s behalf and that the jury should have received clearer instructions on that point.

To many observers, the court essentially said that a politician can be found guilty of corruption only if the government can definitively show an official “quo” in response to a benefactor’s “quid” — a very high bar in a world of winks and nods.

“When you have a system that defines the line between illegal and legal as it does, there are ways of kind of working through it,” Mr. Abramoff said. “Maybe 95 percent or 99 percent of what I did wasn’t really illegal.”

The court’s decision was quietly celebrated by politicians who believe that prosecutors who are intent on criminalizing ordinary political wheeling and dealing have overreached in a number of cases and needed to be slapped down. They worry that too wide a net could eventually ensnare them and their colleagues.

But the ruling was loudly protested by good-government advocates who worried that the court had thrown open the door to more misdeeds and undermined already flagging public confidence in government.

“This is an absolutely terrible message to the public at the worst possible time, when our campaigns are being flooded with huge contributions that are going to buy influence in the future,” said Fred Wertheimer, a longtime campaign watchdog and the president of Democracy 21, a group that pushes for government transparency. “The court forgot about the public.”