As a jury in North Carolina weighs whether John Edwards violated campaign finance law when he attempted to cover up an affair, several legal experts contend that murky questions about the law would offer Edwards a good chance of winning an appeal if he is convicted.

The case centers on nearly $1 million that two campaign supporters allegedly paid for the living expenses and medical bills of Edwards’s mistress, Rielle Hunter, to prevent news of the affair from becoming public.

The crux of the prosecution hinges on a fundamental question about campaign finance law: What is a campaign contribution? The parties in the case agree on the amount of money spent and who spent it — the dispute is over whether it falls under the rubric of a campaign contribution.

The case started with the decision to prosecute Edwards on a novel legal theory that expanded the standard definition of a campaign contribution, campaign lawyers say. In bringing the case, the Department of Justice was setting aside an earlier policy that it would not pursue cases that had been turned down by the Federal Election Commission. A Justice Department spokesman declined to comment.

The government has argued that the payments were made to cover up the affair to protect Edwards’s candidacy for the 2008 Democratic presidential nomination. Prosecutors have said that even payments for personal expenses can be considered contributions if they were made to help a candidate run for office.

Edwards’s lawyers have argued that he was not involved in arranging for the payments.

In arguing that there would be grounds for an appeal, some legal experts cite the decision by U.S. District Court Judge Catherine Eagles barring defense testimony from a former FEC chairman that the commission would not consider the payments to Hunter to be campaign contributions.

The judge also barred Edwards’s campaign treasurer from telling the jury that the commission decided in an audit that the payments were not contributions.

“The FEC is considered to have the special expertise in interpretation of the Federal Election Campaign Act,” said Brett Kappel, an election lawyer at Arent Fox. “It absolutely should have been allowed in.”

The government needs to prove that Edwards knew he was violating the law when he allegedly arranged the payments. But the judge barred testimony that the law was confusing, saying in court that “it just doesn’t seem complicated to me.”

Richard L. Hasen, an election law expert at the University of California, said that precedents and differing interpretations of the law make it complex.

“The judge seems to think it’s fairly straightforward,” Hasen said, “but it seems to me as someone who has studied the law that it’s pretty murky.”

Hasen pointed to complex interpretations from the FEC in the past. In a case in 2000, the commission found that a gift to a candidate would be considered a campaign contribution unless there was already a pattern of giving between the two individuals. In 2002, the commission dismissed a complaint against Rep. James P. Moran (D-Va.), who had received a loan from a drug lobbyist, saying that because the check was signed over to the congressman’s divorce lawyer it had nothing to do with his campaign.

The judge in Edwards’s case instructed the jurors that the law would have been violated if just one of the purposes donor Rachel “Bunny” Mellon gave $700,000 to cover up the affair was to influence the election.

Allyson Hayward, a Republican campaign lawyer, said that in a previous case, the Supreme Court had found that language in the law referring to payments made “for the purpose of influencing” an election was too vague.

“That Bunny Mellon’s own internal ‘purpose’ could mean the difference between freedom and jail for John Edwards is a deeply troubling” circumstance, Hayward said. “If Edwards is convicted in part due to these defective instructions, he has a good chance of winning an appeal.”

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