They were once partners in a lie so spectacular it could have altered the course of a presidential election.

John Edwards, a contender for the 2008 Democratic presidential nomination, and Andrew Young, a once-close personal and political aide to the candidate, allegedly conspired together in an elaborate and expensive scheme to hide Rielle Hunter, Edwards' pregnant mistress, in the midst of the primary campaign.

So devoted was Young, a married man with three children of his own, that he savaged his own future by falsely claiming paternity of Hunter's child, issuing a public statement designed to throw the political press corps off the scent of the scandal. Young says the ruse was Edwards' idea.

"And the most crazy thing about it is," Young told ABC News 20/20, "is that it did work."

That assessment, of course, allows for a generous definition of success.

Edwards continued his campaign but voters in Iowa and New Hampshire rejected his message and turned the primary race into a two-person race between Barack Obama and Hillary Clinton.

By the time Edwards suspended his campaign in late January 2008, Young was in hiding in California with his family and Rielle Hunter, who gave birth to a baby girl in Santa Barbara a month later. Edwards continued to deny he was the father until finally acknowledging paternity in early 2010

Now, as Edwards' criminal trial is set to begin today in federal court in Greensboro, N.C., Young is poised to be the key witness for the prosecution that aims to prove Edwards illegally conspired with Young and others to use hundreds of thousands of dollars in illegal campaign cash to hide Hunter.

Young, who has an immunity deal with prosecutors, has been accused by the defense of siphoning off thousands of dollars of that cash and spending it on himself.

And because both Edwards, 58, and Young, 46, have a history of lying about the events leading up to the trial, the stage is set for some dramatic courtroom moments.

"I think John Edwards' own lawyers are going to have to attack John Edwards," said Dan Abrams, legal analyst for ABC News. "They're going to have to talk about John Edwards as a flawed man and a flawed human being who made some big mistakes."

Similarly, the government will - in some measure - have to go after Young, its most important witness, in order to blunt the impact of what is sure to be an aggressive assault by the defense on Young's credibility.

"This is the guy that publicly claimed that the baby was his," Abrams said. "That makes [Young] an absolutely crucial witness in connection with this case. The defense team is going to go after him.

"They're going to go after his credibility. Why he's saying this, why he's doing this; and how effective the defense is in cross examining Andrew Young could make or break this case."