The following editorial appeared in the Los Angeles Times:

It’s usually difficult to prove beyond a reasonable doubt that a campaign contribution influenced a public official to take an official action. That’s why other ways to limit the influence of money in politics, such as disclosure rules and limits on contributions, are so important. But sometimes the link between cause and effect is so clear that a politician can be convicted of criminal bribery. The Supreme Court last week wisely refused to make such convictions harder to achieve.

The justices refused to hear the appeal of former Alabama Gov. Don Siegelman, who was convicted of reappointing a health-care executive named Richard Scrushy to a state hospital board as a reward for Scrushy’s $500,000 contribution to a campaign fund backing a statewide lottery to fund education.

Although Siegelman didn’t personally benefit from Scrushy’s largesse, the lottery was one of his political priorities and its approval by the voters would have burnished his reputation. For Scrushy, an appointment to the hospital board was important because that body was responsible for determining the number of health-care facilities in the state.

The Siegelman case attracted national attention partly because the former Democratic governor claimed that his prosecution by the Justice Department under President George W. Bush was politically motivated. (The Obama administration, however, defended the verdict.) But Siegelman’s conviction also was criticized for criminalizing business as usual in politics.

Prosecutorial overreach in white-collar and political corruption cases is a real concern, and the Supreme Court needs to be vigilant about checking excesses. It did that in 2010 when, in a case involving a former executive of Enron Corp., it narrowed the definition of a law making it a crime to “deprive another of the intangible right of honest services.” The justices ruled that the law could be used only against defendants who had taken part in bribes or kickbacks.

Siegelman’s bribery conviction didn’t require the same scrutiny. The cornerstone of his appeal was that the government didn’t prove that the governor made an “explicit promise” to reappoint Scrushy in exchange for a contribution. But the U.S. 11th Circuit Court of Appeals concluded convincingly that the jury could have inferred the existence of such an agreement from the testimony it heard, including that of a former Siegelman aide who discussed the matter with the governor.

Not every example of a politician deciding to pay off a campaign contributor will be provable in court. That is lamentably the case with the bipartisan presidential practice of rewarding campaign contributors with ambassadorships. But where the prosecution can amass enough evidence to convince a jury that an official action has been bought and paid for, the verdict should stand.