GREENSBORO, North Carolina (AP) - Prosecutors called a once-trusted aide of John Edwards to the witness stand Monday as they opened their criminal case against the former presidential candidate on charges of violating campaign finance laws to cover up an extramarital affair.

Andrew Young is the linchpin of the government's case against Edwards, a former Democratic party star facing allegations that he masterminded a conspiracy to use nearly $1 million in secret payments from two wealthy donors to help hide his pregnant mistress as he sought the White House in 2008.

Young and Edwards were so close that when Edwards got his mistress pregnant in 2007, the married Young publically claimed paternity of his boss' unborn child. "We were just North Carolina boys and had a lot in common," Young testified.

Edwards, 58, has pleaded not guilty to six criminal counts related to alleged violations of federal campaign finance laws stemming from accepting money in excess of the $2,300 legal limit for individual contributions. Federal law defines campaign contributions as money given to influence the outcome of a U.S. election.

"It wasn't just a marriage on the line," prosecutor David Harbach said in his opening statement at the trial in Greensboro, North Carolina. "If the affair went public it would destroy his chance of becoming president, and he knew it. ...He made a choice to break the law."

Edwards, a former North Carolina senator, stared intently at Young as his former confidant testified. In nearly two hours of talking about Edwards, Young never looked in his direction.

For Edwards and his defense team, destroying Young's credibility is key to their strategy of keeping the former presidential contender out of prison.

They allege that much of the money at issue in the case was siphoned off by Young and his wife to pay for a $1.5 million house finished in 2008.

"Follow the money," defense lawyer Allison Van Laningham urged jurors in her opening statement. "John Edwards did not get any of this money. Not one cent."

Edwards' lawyers contend the payments were gifts from friends intent on keeping the candidate's wife from finding out about the affair. Elizabeth Edwards died in December 2010 after battling cancer.

A key issue will be whether Edwards knew about the payments made on his behalf by his national campaign finance chairman, the late Texas lawyer Fred Baron, and campaign donor Rachel "Bunny" Mellon, a now-101-year-old heiress and socialite. Each had already given Edwards' campaign the maximum $2,300 individual contribution.

Edwards denies having known about the money, which paid for private jets, luxury hotels and then-mistress Rielle Hunter's medical care. Prosecutors will seek to prove he sought and directed the payments to cover up his affair, protect his public image as a "family man" and keep his presidential hopes viable.

Young testified about first meeting Hunter as she travelled with Edwards in 2006. That same year, Young first spoke with Mellon and put her in touch with Edwards.

The Youngs later invited the pregnant Hunter to live in their home and embarked with her on a cross-country odyssey as they sought to elude tabloid reporters trying to expose the candidate's extramarital affair. Edwards and Young eventually had a falling out and the former aide wrote an unflattering tell-all book, "The Politician."

After years of adamant public denials, Edwards acknowledged paternity of Hunter's daughter, Frances Quinn Hunter, in 2010. The girl, now 4, lives with her mother.

It has not yet been decided whether Edwards, a former trial lawyer once renowned for his ability to charm jurors, will testify in his own defense.

Before the jury entered the courtroom Monday, U.S. District Court Judge Catherine C. Eagles disclosed that Young had called three other witnesses in the last two weeks. Eagles ruled that lawyers for Edwards could mention the improper contact to jurors in opening statements Monday, but barred them from using the term "witness tampering" or telling the jury that Young had a one-night stand with one of the other witnesses in 2007.

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