The prosecution's key witness in the case against John Edwards prepared to take the stand for the second day Tuesday, even as the former Edwards confidante began to pose potential problems for the government's case. 

The testimony of former aide Andrew Young is supposed to be a linchpin in the government's claim that Edwards knew two wealthy donors had contributed nearly $1 million to hide his pregnant mistress -- and in doing so violated federal campaign finance law. Young hinted at this narrative when he took the stand Monday on the opening day of arguments. 

But U.S. District Court Judge Catherine Eagles disclosed Monday that the former Edwards aide had recently contacted three other witnesses in the trial to ask what they planned to say. In a meeting with lawyers before the jury entered the room, Eagles also noted that Young had a one-night stand with an unidentified witness in 2007 -- the judge ruled the lawyers could not mention the one-night stand to the jury, but could mention the improper witness contact provided they didn't describe it as "witness tampering." 

Young already was entering the trial with some questions in his background -- he was the aide who initially and falsely claimed he was the father of Edwards' illegitimate child. 

Kieran Shanahan, a Raleigh lawyer and former federal prosecutor who is attending the trial, said the prosecution is "taking a bit of a risk" by putting their star witness on first. 

"If in cross-examination they're able to destroy his credibility, it'll be very difficult for the government to recover," Shanahan said. 

Prosecutors are trying to prove that the funds in question were not just private gifts to hide Edwards' affair from his cancer-stricken wife, but actual campaign contributions intended to prevent the affair from damaging his bid for president. They plan to play voicemails Edwards left on Young's phone during the campaign. 

The defense claims the secret payments were not intended to influence the outcome of the election, but to spare Edwards' family pain and embarrassment. Edwards' lawyers also allege that Young used much of the money to build an upscale house for himself in Chapel Hill, N.C.

Young began to spill details Monday of the people who surrounded Edwards and put their faith in him. 

The former campaign worker, who testified in exchange for immunity, told the court Monday about a meeting he attended with one of the two wealthy Edwards supporters, Rachel "Bunny" Mellon. 

"I'm going to do everything I can to help you become president of the United States," Mellon told Edwards, according to Young. 

Young also said Mellon thought Edwards would be the country's "savior." 

On the witness stand Monday, Young recounted how he met Edwards in 1998, as the Raleigh trial lawyer and political neophyte was campaigning for the Senate. 

"My father was a minister, so I had seen a lot of great speakers," Young recounted. "He was really `on' that day." 

Young said he immediately told his future wife, Cheri Young, that Edwards had the potential to become president and that he wanted to work for him. Young quickly rose from a junior campaign staffer to working on the senator's North Carolina staff following the election. When no one else wanted to pick up Edwards at the airport, Young leaped at the opportunity. He eventually became special assistant to the senator, a gatekeeper who got phone calls and face time with Edwards. 

Young also testified about first meeting Rielle Hunter as she traveled 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 near Chapel Hill and embarked with her on a cross-country odyssey as they sought to elude tabloid reporters trying to expose the candidate's extramarital affair. 

The day of opening arguments comes more than four years after Edwards ended his Democratic presidential bid and went on to watch his reputation crumble under the weight of the emerging scandal. He has gone from battling reporters to battling prosecutors who seized upon the scheme as an alleged campaign finance breach. While the lurid details of his affair dominated the headlines going into the summer of 2008, the focus of the trial is the money trail. 

Edwards faces six criminal counts of campaign finance violations for allegedly accepting, and failing to report, campaign donations in excess of the $2,300 limit for individual contributions. If convicted, he could face up to 30 years in prison and $1.5 million in fines. 

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