The indictment of two-time presidential candidate John Edwards on Friday for conspiring to cover up gifts he accepted to obscure an extra-marital affair could signal the Justice Department’s desire to step up enforcement of campaign finance law.

But legal experts say securing a conviction could be a bumpy road.

There are significant roadblocks prosecutors must overcome to make the case that the former Senator  knowingly and willfully violated campaign finance law — the standard of proof required in a criminal case — given that the Federal Election Commission has issued conflicting opinions about in which situations money given to candidates is a personal gift or a campaign contribution, legal experts said last week.

“This is an unprecedented prosecution … no one would have known, or should have known, or could have been expected to know, that these payments would be treated or should be considered as campaign contributions. And there was no way Sen. Edwards knew that fact either,” Edwards’ lead counsel, Gregory Craig, said outside the Winston-Salem courthouse following the indictment. Edwards pleaded not guilty.

Edwards, 57, was indicted by a federal grand jury Friday on charges of knowingly receiving and concealing contributions of more than $925,000 from two wealthy donors from 2007 to 2009. They are not named in the documents but are known to be Rachel “Bunny” Mellon, the 100-year-old widow of billionaire philanthropist Paul Mellon, and Fred Baron, a Texas trial lawyer and prominent Democratic fundraiser at that time. The money was used to pay living and travel expenses for Edwards’ mistress, videographer Rielle Hunter.

Edwards’ legal team says the campaign committee did not report these donations, which were well in excess of the $2,300 legal limit on individual contributions for the primary, because they were private gifts intended to keep his mistress and their child a secret from his wife, Elizabeth, who died of cancer in December. Federal prosecutors believe those donations should have been reported publicly as campaign contributions because Edwards’ bid for the Democratic nomination relied in part on his image as a devoted father and family man.

The Edwards prosecution will be the most high-profile case brought by the Justice Department’s Public Integrity Section since the botched prosecution of former Sen. Ted Stevens, who died in a plane crash in 2010. Though a jury convicted the Alaskan of lying about gifts on his Senate financial disclosure forms, a judge voided the verdict against the Republican lawmaker after finding the prosecution team had failed to disclose evidence to the defense as required by law.

Experts say that for the Justice Department to secure a conviction of Edwards, it would require a broad reading of what constitutes a campaign contribution, which is further complicated by conflicting precedent set in similar civil cases considered by the FEC.

The prosecution is said to rely in part on an FEC advisory opinion from 2000 that said an individual could not give money to a candidate for “personal use” if it was donated in “recognition and support of that person’s desire to run for office.” But two years later, the commissioners dismissed a case against a Representative who used a loan from a lobbyist to pay for a divorce attorney.

“There are a number of gray areas in campaign finance law that require the FEC to issue an advisory opinion,” said Rick Hasen, law professor at University of California, Irvine. “And it’s quite problematic to use a gray area for a criminal prosecution.”

Edwards’ legal team already has experts steeped in the intricacies of campaign finance law at the ready. In addition to Edwards’ trial lawyers in North Carolina and D.C., former FEC Chairman Scott Thomas and Pat Fiori, the commission’s former counsel for regulations and legislation, are acting as expert witness and campaign counsel.

Thomas has already told federal prosecutors that after 30 years at the FEC he does not believe there are any prior cases similar to Edwards’ that conclude the conduct violated campaign finance law. Thomas, as a commissioner, signed the advisory opinion on which the government has built its case and said its application to the Edwards matter is “novel and misguided.”

“I believe that the theory on which the government intends to base its prosecution is without precedent in federal election law, and that the Federal Election Commission would not support a finding that the conduct at issue constituted a civil violation much less warranted a criminal prosecution,” Thomas said in a statement released to the media.

Edwards likely made a tempting target for the Justice Department, given the nature of the allegations and the immobilization within the FEC, which often splits 3-3 on matters under its consideration. An investigation into civil wrongdoing cannot commence without the approval of the commissioners. Unlike a Justice Department investigation, which relies on interviews of key witnesses, an FEC inquiry typically relies on letters sent from lawyer to lawyer.

“The differences are night and day,” said Caplin & Drysdale attorney and former FEC Chairman Trevor Potter.

Though criminal prosecutors have more tools at their disposal, there are drawbacks to using criminal statutes to bring cases against Edwards or former Sen. John Ensign (R-Nev.), whose case — also involving payments to his mistress — was recently referred to the Justice Department by the Senate Ethics Committee, due in part to allegations of campaign finance violations.

“As a civil case, I think the case would be stronger against Edwards, in a criminal case, you need to meet the standard of proof, which is beyond a reasonable doubt,” Hasen said. “And the fact that they have to find FEC commissioners to testify as to what the law is means the law is murky.”

Janie Lorber contributed to this report.