Andrew Young was once much more than an aide to Edwards. He testified after opening statements that he and Edwards were "just North Carolina boys and had a lot in common." They were so close that when Edwards' mistress, Rielle Hunter, got pregnant in 2007, Young publically claimed paternity.
Prosecutors allege that Edwards, 58, 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.
"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," Prosecutor David Harbach said in his opening statement.
The two-time Democratic presidential candidate 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.
His lawyer, Van Laningham, said Edwards' public fall from grace has humbled the former politician. He lied to everyone, she said - his wife, his staff and the American people.
"John Edwards is a man who committed many sins," she told the jury, "but no crimes."
Edwards stared intently at Young as his former aide testified. In nearly two hours of talking about Edwards, Young never looked in his direction. The two had a falling out, and the former aide wrote an unflattering tell-all book, "The Politician."
But Young is not a perfect witness for prosecutors. The judge revealed that Young improperly contacted other witnesses before the trial and had a one-night stand with one of them in 2007.
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.
Edwards denies having known about the money. It has not yet been decided whether Edwards will testify in his own defense.
The Associated Press contributed to this report.