John Edwards

* Prosecutors say more than $900,000 in campaign funds illegal

* Money used to cover up affair, pregnancy of mistress

* Edwards was married 2008 Democratic presidential hopeful

By Colleen Jenkins

WINSTON-SALEM, N.C., April 22 (Reuters) - Former U.S. Senator John Edwards goes on trial Monday on charges he used illegal campaign contributions to cover up an affair with a mistress who became pregnant during his failed bid for the 2008 Democratic presidential nomination.

Edwards is accused of accepting more than $900,000 in campaign funds from two wealthy donors, knowing the exposure of his extramarital affair "would destroy his presidential campaign," prosecutors said in a trial brief.

The candidate at the time was a married father of three, whose late wife, Elizabeth, had breast cancer.

Jurors will hear opening statements the federal courthouse in Greensboro, North Carolina.

Edwards, 58, is accused of conspiring to solicit the money, receiving more than the $2,300 allowed from any one donor, and failing to report the payments as contributions.

He faces six felony counts, each carrying a sentence of up to five years in prison and a $250,000 fine.

Edwards admits personal failings but insists he broke no laws.

Edwards' defenders say the government is overreaching with its prosecution of Edwards, the son of mill workers who earned his fortune as a trial lawyer in North Carolina before being elected as a U.S. senator from the state in 1998.

His defense lawyers dispute the Justice Department's interpretation of federal election laws, arguing the donors would have given the money regardless of the campaign and did so knowing it wouldn't be used for campaign purposes.

The money was not spent to influence the election but rather to conceal the affair and resulting pregnancy from Edwards' wife and children, they said.

Edwards never personally received any of the payments, nor did his campaign. The money was used to cover living expenses and medical care for his mistress, campaign videographer Rielle Hunter, rather than traditional campaign activities.

"This is expanding the scope of the definition of campaign contribution," said Ron Wright, a law professor at Wake Forest University who is not involved in the case. "It is an unprecedented definition."


PIVOTAL POINTS

The defense is expected to call two former Federal Election Commission members who, if allowed by the judge, would testify they believe Edwards did not violate campaign finance laws.

Hampton Dellinger, a former deputy attorney general who has followed the Edwards case, said the campaign finance experts' testimony could be pivotal.

The missing pieces of the case also could be significant, he said. Neither of the two donors whose payments are in question are able to testify.

Fred Baron, who served as Edwards' national campaign finance chairman in 2008, has since died, and heiress Rachel "Bunny" Mellon is 101 and physically unable to attend the trial. Elizabeth Edwards died in 2010.

A chief government witness will be Andrew Young, a campaign aide who later wrote a tell-all book about Edwards' affair and the efforts to keep it out of the public eye.

Young, who was granted immunity, initially claimed he had fathered a child with Hunter, who is also expected to testify. A lawsuit settlement earlier this year between Young and Hunter called for copies of a videotape purported to show her having sex with Edwards to be destroyed.

Edwards' defense team, recently reshuffled to include the lawyers who represented Hunter in the civil case, has indicated it will attack Young's motives and credibility.

Edwards, who also ran for president in 2004 before becoming John Kerry's vice presidential running mate the same year, has his own credibility issues. He repeatedly denied having an affair and daughter with Hunter, and finally admitted paternity two years after the child's birth.

"This case is not so much the United States v. John Edwards, it's Andrew Young v. John Edwards," Dellinger said. "And I think the jury's determination about which one of them is more credible may be one of the key factors in deciding whether Mr. Edwards is guilty."

The trial could last until late May or early June. A conviction would make what qualifies as a campaign contribution less certain for future candidates, said law professor Wright.

"It's going to mean lots more lawyers employed by campaigns," he said. "There's going to be a lot more legal risk involved in election reporting if the government wins this." (Editing by Daniel Trotta and Doina Chiacu)