Mr. Bookbinder added that, using Mr. Silver’s case as a guide, it may be possible to map the boundaries of how courts may define corruption. “It looks like a politician will now have to take a very formal action” — such as casting a favorable vote for a donor or awarding a government contract to a friend — “for it to be considered illegal,” he said. “The problem is that a lot of political power is wielded less formally than that.”

In many ways, the reversal of Mr. Silver’s conviction was based on a quirk of timing: Mr. McDonnell’s case made its way to the Supreme Court in the midst of the former Assembly speaker’s legal ordeal. The appeals court said in its ruling that there was sufficient evidence to convict Mr. Silver, but it chose to dismiss the guilty verdict because the jury had not been instructed on the new definition of corruption established by the Supreme Court.

There could be profound consequences for other elected officials who have been found guilty of graft or are facing trial on corruption charges. Among them are Dean G. Skelos, the former Republican majority leader of the New York State Senate who was sentenced last year to five years in prison for using his office to assist his son; Chaka Fattah, a former Democratic congressman from Philadelphia who was convicted last year on bribery, money laundering and racketeering charges; and Senator Robert Menendez, a New Jersey Democrat who is scheduled to go on trial in the fall on charges that he traded political favors for luxury vacations, campaign donations and expensive airline flights.

Some experts said the McDonnell ruling would probably not extend to old corruption cases, adding that it was difficult to reverse guilty verdicts that had already been appealed. Others noted that the new legal landscape could put a chill on future corruption investigations.

In March, for instance, the United States attorney’s office in Manhattan, which prosecuted Mr. Silver and Mr. Skelos, said it would not seek charges against Mayor Bill de Blasio of New York after a monthslong inquiry into whether his administration had traded favorable government actions for political donations.

In a statement at the time, the office said it had found a pattern in which the mayor or his associates had solicited contributions from donors seeking favors from the city, but had decided not to bring a case after weighing, among other things, the “high burden of proof” created by the McDonnell ruling.