A key prosecution witness in the criminal trial of former presidential candidate John Edwards contacted other witnesses in the case to ask about their planned testimony, a possible violation of federal law, a judge said Monday.

U.S. District Court Judge Catherine C. Eagles said that former Edwards aide Andrew Young called the 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.

Young once falsely claimed paternity of the child Edwards fathered with his then-mistress Rielle Hunter in 2007. The former aide is potentially the government's most important witness as prosecutors seek to prove the then-married Democratic candidate masterminded a scheme to use nearly $1 million provided by two wealthy campaign donors to help hide his pregnant mistress as he sought the White House in 2008.

Edwards, 58, has pleaded not guilty to six criminal counts related to alleged violations of federal campaign finance laws.

The former candidate sat silently in the courtroom Monday morning as the lead federal prosecutor called him a man who would say or do anything to get elected president, including violating the law to hide his affair and keep his campaign viable.

"It wasn't just a marriage on the line," prosecutor David Harbach said. "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."

The judge seated 12 jurors and four alternates Monday morning. The panel is made up of nine men and seven women drawn from central North Carolina, the state that elected Edwards to serve one term in the U.S. Senate.

A defense lawyer for Edwards told the jury that most of the money at issue in the case went not to support Hunter, but was siphoned off by Young and his wife to build a $1.5 million "dream home" near Chapel Hill. 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 allowed by federal law.

Edwards denies having known about the money, which paid for private jets, luxury hotels and 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 and his wife invited the pregnant Hunter to live in their home near Chapel Hill and later embarked with her on a cross-country odyssey as they sought to elude tabloid reporters trying to expose the candidate's extramarital affair.

Young later fell out with Edwards and wrote an unflattering tell-all book, "The Politician." Young and Hunter recently ended a two-year legal battle over ownership of a sex tape the mistress recorded with Edwards during the campaign, agreeing to a settlement that dictates that copies of the video will be destroyed.

Young is expected to be a witness for the prosecution, while the defense is likely to call Hunter to testify. Two of the lawyers who represented Hunter in her civil suit against the former aide joined Edwards' legal team last month. 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 in Charlotte.

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.

Eagles, who was appointed in 2010 by President Barack Obama, said she expects the proceedings to last about six weeks.

Eagles last week ordered a former speech writer for Edwards to turn over any drafts of books or articles involving him and other key players in his case. Edwards' lawyers are seeking an extensive list of documents from Wendy Button, a 2008 campaign staffer expected to testify.

The judge ruled that Button did not have to turn over all communications with third parties including the media regarding Edwards, Hunter and Young from 2009 to the present. Button also doesn't have to produce correspondence or proposals involving books and articles involving Edwards and key players in the case, or turn over postings on Facebook or other social media sites regarding Edwards during that time period.

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Follow AP writer Michael Biesecker at: http://www.twitter.com/mbieseck