The trend began in 1999 when a Supreme Court case called United States v. Sun Diamond Growers of California chipped away at the government’s ability to prosecute officials for taking what are known as gratuities — or minor gifts given to them by businesses or allies. The opinion found that gratuities were illegal only if the government could connect the gifts to specific favors by officials, establishing a visible quid pro quo.
In 2010, the court attacked another anti-corruption tactic, narrowing the definition of what is known as honest services fraud. The ruling in this case came as the justices reversed parts of the criminal conviction of Jeffrey K. Skilling, the former Enron chief executive who had been found guilty of charges related to his company’s collapse. Although Mr. Skilling was a private citizen, the opinion had a political effect: the newly limited fraud law had frequently been used to go after politicians who served themselves at their constituents’ expense.
But the court’s most substantial opinion on corruption came last year when it redefined the very nature of political graft in throwing out the bribery conviction of Bob McDonnell, the former Republican governor of Virginia. A jury determined that Mr. McDonnell had helped a wealthy businessman by setting him up with influential people in an effort to promote a dietary supplement he was selling. But even though the businessman had given the governor several gifts and loans, the court concluded it was not illegal. It ruled that Mr. McDonnell’s part of the arrangement — making introductions and setting up meetings — was not in fact a betrayal of his office, or what the law describes as an “official act.”