Federal prosecutors seeking to prove that John Edwards used campaign funds to hide his pregnant mistress during his 2008 presidential run will have their work cut out for them.

Two crucial witnesses are dead. Another is 100 years old. A fourth, former Edwards aide Andrew Young who initially claimed the baby was his before coming clean in a tell-all book, was recently held in contempt of court.

And the government's case revolves around an untested legal theory that the $950,000 given to the two-time presidential candidate by two of his close friends to hide his affair should have been considered a campaign contribution.

Edwards' attorneys counter with an argument that's reprehensible but could raise reasonable doubts with a jury: He was only interested in hiding the affair from his cancer-stricken wife, who died in December.

The stakes couldn't be higher for a Justice Department section still trying to recover from botching its last major political case against the late Sen. Ted Stevens, R-Alaska, whose guilty verdict was vacated after the department admitted it withheld key evidence from the senator's legal team.

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The six-count indictment accuses Edwards of conspiracy, taking illegal campaign contributions and making false statements. He faces up to five years in prison and a $250,000 fine on each count if convicted. On Friday, appearing both defiant and contrite, he insisted he did not break the law.

Some legal experts tend to agree.

"It is surprising that the Department of Justice is following the bungled prosecution of now-deceased Sen. Ted Stevens with this remarkably weak case against former Sen. John Edwards, said Melanie Sloan, executive director of the Citizens for Responsibility and Ethics in Washington, which typically criticizes the Justice Department for not pursuing enough cases against public officials.

"Like the Stevens case, the Edwards matter is likely to leave DOJ with egg on its face."

Sloan, a former federal prosecutor, called the government's entire case rests on finding that the payments made by Mellon and Baron were campaign contributions.

"But no court has ever interpreted the definition of campaign contribution this broadly," she said. "Further, in his book, Mr. Young claimed that those payments were entirely proper gifts."

The federal investigation focused particularly on money coming from two Edwards supporters -- former campaign finance chairman Fred Baron and Rachel "Bunny" Mellon, the widow of banking heir Paul Mellon. Combined, they provided $950,000 to help hide Hunter, according to the indictment.

Prosecutors contend the money was intended to aid his campaign by preventing public disclosure of the affair, which would have destroyed his candidacy. But prosecutors could have difficulty proving intent, given that Baron died in 2008 shortly after the scandal surfaced and Mellon is now 100 years old.

Young will likely be a key government witness. Edwards' attorneys have already tried to portray him as being motivated by financial gain.

Hunter sued Young around the time he was releasing a tell-all book, and her attorneys aggressively questioned his credibility. A North Carolina judge said he was troubled by a series of seemingly conflicting statements Young made under oath in the lawsuit involving a purported sex tape depicting Edwards. Superior Court Judge Abraham Penn Jones held Young in contempt and considered sending him to jail before backing away as Young's lawyers argued that the discrepancies were memory lapses.

Prosecutors did cite some evidence in their indictment that could be used to argue that participants of the conspiracy knew the money was going to aid Edwards' candidacy. A note from Mellon in 2007 indicated that she wanted to help pay Edwards' bills "without government restrictions."

In another section, prosecutors claim Edwards told a former aide that he was aware of Baron's payments even though he publicly claimed otherwise.

White-collar attorney Matt Miner said although the four counts of receiving illegal campaign contributions may be novel applications of the law, the false statement and conspiracy charges are more straightforward and could be the most problematic for Edwards.

"If Edwards can be shown to have conspired to receive contributions in a way that he would not otherwise have to report -- and then those false reports were filed under his signature -- both charges are fairly straightforward conspiracy and false statement charges," Miner said.

Edwards initially denied having an affair with Hunter both during his candidacy and after his campaign ended. Baron continued his support of Young and Hunter after the campaign ended as well, adding support to the argument that the money wasn't necessarily related to his candidacy.

In August 2008, Edwards admitted the affair. But he denied fathering Hunter's child until the beginning of last year.

"Sen. Edwards' conduct was despicable and deserves society's condemnation, but that alone does not provide solid grounds for a criminal case," Sloan said. "DOJ's scattershot approach to prosecuting public officials is incomprehensible and undermines the integrity of the criminal justice system."

The Associated Press contributed to this report.