Ex-aide testifies how cover-up of Edwards' affair unraveled, along with their friendship

It was the worst, best-kept secret of the 2008 presidential election cycle, and in the end it tore apart the friendship of John Edwards and his most trusted aide.

That aide, Andrew Young, is now the key witness against Edwards in the failed Democratic presidential candidate's trial on charges he used campaign donations to cover up an affair, and on Wednesday, Young testified that Edwards' behavior became increasingly unpredictable as the lie unraveled.

With Young's frustration growing the deeper he got into the cover-up, he and Edwards had a long-awaited and contentious face-to-face meeting on June 18, 2008.

"He told me that he loved me and that I knew he would leave me," Young said in his third day of testimony, but Young didn't feel Edwards was answering his questions about the cover-up.

Edwards' mistress, the campaign videographer Rielle Hunter, was living with Young and his wife after Young agreed to pass Edwards' love child off as his own, shielding the truth from Edwards' wife, Elizabeth Edwards. But Young testified that the pressure was mounting on the former North Carolina senator, as National Enquirer reporters staked Edwards out while he was visiting Hunter and the child at a California hotel.

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Edwards is accused of conspiring to use secret payments from two wealthy donors to hide Hunter during his White House run. Edwards has pleaded not guilty to six counts related to campaign finance violations.

If convicted on all six counts, Edwards faces a maximum penalty of 30 years in prison and as much as $1.5 million in fines.

Edwards has denied knowing about the money, much of which flowed into accounts controlled by Young and his wife, Cheri. Edwards' lawyers claim the Youngs siphoned off the bulk of the money to pay for their $1.5 million house near Chapel Hill.

Edwards allegedly directed Young to start giving money to Hunter in May 2007, after she threatened to go to the media and expose the affair. Edwards suggested asking elderly heiress Rachel "Bunny" Mellon, who had already given generously to the campaign.

Prosecutors showed the jury checks from Mellon written to her interior designer, who would then endorse them and send them to Young and his wife. Starting in June 2007, Mellon would eventually provide checks totaling $725,000, funds that Young said Edwards and he called the "Bunny money."

Telling Mellon the money would be used for a "non-campaign" expense, Young said she offered to provide $1.2 million over time to help. Under federal law, donors are limited to giving a maximum of $2,300 per election cycle.

Young said Edwards stopped returning his calls in January 2008, as Edwards was suspending his White House bid after early primary losses in Iowa and New Hampshire.

The prior month, Edwards issued a false statement to the press, with Young claiming the child was his. Edwards said he would set the record straight after the baby was born, according to Young.

Yet months passed with no call from Edwards, and Young said he and his wife had grown tired of sharing a house with the increasingly-demanding mistress.

Young testified that he flew to Texas in May 2008 with his wife to meet with one of the wealthy donors, Fred Baron, with four demands. He wanted to know why Edwards hadn't claimed paternity; he wanted a face-to-face meeting with him; he wanted to know what his long-term plans were and he wanted to stop living with Hunter.

Baron arranged for Hunter to move out of a Santa Barbara, Calif., house she was sharing with the Youngs, and he set up the meeting. But before that happened, Edwards did eventually call Young and leave a voicemail. It was played in court: "I miss talking to you Andrew. We'll see you pal."

Edwards' political hopes dimmed that July when the National Enquirer reporters photographed him at the California hotel with his mistress and baby daughter, Frances Quinn Hunter, who was then 5 months old.

Edwards went on national TV and again denied having an affair with Hunter or fathering her child. It would be another two years before Edwards finally admitted paternity. The girl, now 4, lives with her mother in Charlotte.

Young testified that when he saw the interview, he became incensed. He demanded another face-to-face meeting, which occurred in August 2008.

Young said he told Edwards he had "evidence of everything that had happened."

"He looked at me and he said, 'You can't hurt me, Andrew. You can't hurt me,'" Young said.

It was the last conversation of any substance they would have.

Fox News' Jonathan Serrie and the Associated Press contributed to this report.