WASHINGTON — Former Alabama Gov. Don Siegelman was charged with bribery and sent to prison because, prosecutors said, a wealthy hospital executive gave him $500,000 in exchange for appointing him to a state hospital planning board.

But this half-million-dollar “bribe” did not enrich Siegelman. Instead, the disputed money was a contribution to help fund a statewide referendum on whether Alabama should have a state lottery to support education, a pet cause of the governor’s.

The Supreme Court is set to decide as soon as Monday whether to hear Siegelman’s final appeal, which raises a far-reaching question: Is a campaign contribution a bribe if a politician agrees to do something in return, or is it to be expected that politicians will do favors for their biggest supporters?

Prominent election law experts and more than 100 former state attorneys general have urged the justices to review Siegelman’s case. They say the law in this area is hazy, with the result that aggressive prosecutors can bring charges against political enemies.

Siegelman was the rare Democrat who could win in Alabama. He had also won election as Alabama’s secretary of state, attorney general and lieutenant governor. But his career ended when Republican-appointed U.S. attorneys charged him with corruption.

Siegelman’s supporters noted that Leura Canary, the U.S. attorney in Montgomery, was the wife of William Canary, a prominent GOP operative and an ally of Karl Rove, President George W. Bush’s chief political strategist.

The legal uncertainty over campaign contributions also figured in the trial of former Sen. John Edwards, D-N.C. He was charged with campaign finance violations after two wealthy friends spent more than $900,000 helping hide his mistress. On Thursday, a jury acquitted him on one count and deadlocked on the remaining charges.

Washington lawyer Sam Heldman, who represents Siegelman, says the Supreme Court should rein in the use of bribery laws when politicians are not accused of taking money for themselves. “If campaign contributions are to be treated as bribes, Congress ought to write a law that says that,” he said.

The federal bribery law makes it a crime for an official to “corruptly solicit, demand ... or accept ... anything of value of $5,000 or more” with the intent of being “influenced or rewarded.” The law was aimed at officials who secretly take money for themselves, but prosecutors have used it against officials who use their offices to solicit campaign contributions.

But the line between legitimate political fundraising and criminal bribery is none too clear. “It’s an extraordinarily difficult problem,” said University of California, Los Angeles law professor Daniel H. Lowenstein, who has written extensively on the issue.

The Supreme Court has not ruled directly on when campaign contributions can be considered bribes, but in a related case, the justices said in 1991 that a state legislator could not be convicted of extorting contributions unless it was proved he made an “explicit promise” to introduce a bill in exchange for money.

The Alabama case began with Richard Scrushy, the high-flying founder of HealthSouth Corp., once the nation’s largest chain of outpatient healthcare facilities. He had given hundreds of thousands dollars to support three Republican governors and had been named by each to sit on a state hospital planning board.

After Siegelman won election, aides to the two men met and reportedly agreed Scrushy could win favor with the new governor by giving a similar amount. Scrushy wrote a first check of $250,000 to support the lottery campaign, and a week later, Siegelman reappointed him to the hospital board.

But Scrushy’s lavish lifestyle drew attention, and in 2003, the Justice Department accused him of a massive accounting fraud. He was indicted on 85 counts of fraud, conspiracy and money laundering. To the surprise of many, a jury in Birmingham acquitted him on all the counts in 2005.

A year later, however, prosecutors in Montgomery charged Scrushy and Siegelman with bribery, alleging the $500,000 payment to the lottery campaign fund bought a seat on the hospital board. In 2006, a jury acquitted the pair on most of the charges and convicted them of bribery. Although Siegelman was not convicted of taking money for himself, prosecutors asked for a 30-year prison sentence.

The judge gave both men seven years in prison, and they were taken away in shackles to begin serving their time. In 2008, after widespread complaints about Siegelman’s trial, a U.S. appeals court freed him while he appealed.

The 113 former state attorneys general who supported his appeal said they worried about “arbitrary and abusive enforcement of the law,” particularly when campaign contributions are deemed as bribes. Justice Department lawyers urged the court to turn down the appeal, saying the jury had heard the evidence and concluded the pair had made an illegal, explicit deal.