Former U.S. Senator John Edwards arrives with his daughter Cate Edwards at the federal courthouse in Greensboro, North Carolina April 23, 2012. Edwards, 58, is accused of accepting more than $900,000 in illegal campaign funds from two wealthy donors to hide his pregnant mistress during his failed bid for the 2008 Democratic presidential nomination. (CHRIS KEANE/REUTERS)

Of course, as the prosecution stressed during opening arguments in the case against the two-time presidential candidate, Johnny Reid Edwards is not on trial for two-timing his wife, who died of cancer in 2010.

But the criminal case against the former senator does turn on whether he knowingly violated the law by using more than $900,000 in gifts from two wealthy donors to keep his ‘08 presidential hopes alive by covering up his affair with Rielle Hunter.

“It’s often said ambition is an important quality in a politician,’’ prosecutor David Harbach began. “But like many things in life, too much can be dangerous.” In pursuing his ultimate ambition, the prosecutor said, Edwards didn’t care who he hurt or what he broke: “not other people, not family, not the law.’’

“In the heat of his second race’’ for the top office, Harbach said, “he was also in the heat of an affair’’ with Hunter, whom he hired to join the campaign so they could more conveniently travel together. But then, the prosecutor charged, he became desperate to keep her quiet, especially after she became pregnant, “because his mistress was a loose cannon with no source of income and he knew it.”

Prosecutors have to prove Edwards knew the law, and intended to break it: “He made a choice to accept hundreds of thousands and he made a choice to break the law; that’s why we are here,’’ said Harbach, and the defendant’s grown daughter Cate, seated behind him in the courtroom, crossed her arms over her chest.

His defense suggested that that law is so complicated it will be hard to prove he even knew what it was.

Physically, the defendant is curiously unaltered from the boyish-looking man who talked about “the two Americas” of haves and have-nots in 2004; he has only a few gray hairs, and still wears the Bruno Maglis he favored on the trail. Though he isn’t, as he always used to, wearing his late son Wade’s Outward Bound pin on his left lapel, over his heart.

The former trial lawyer is clearly an active participant in his own defense, constantly conferring with his attorneys at the defendant’s table during the final phase of jury selection. He was openly sizing up potential jurors and repeatedly beckoning Cate, who like both of her parents went to law school, to come close so he could tell her something.

Also in the courtroom were his elderly parents, seated on cushions on the wooden benches. His father, whose humble roots as a millworker in a small town not far from here were a staple of Edwards’s stump speech, warmly greeted his son, patting both of his arms. At one point before the jury came in, Edwards got a Styrofoam cup of water for his mother.

Aside from the jurors and alternates — 9 men and 7 women — most of those filling the courtroom are journalists.

No, this isn’t Watergate, but Edwards’ defense attorney repeatedly asked the jurors to “follow the money,’’ – and promised that if they did, they’d see that it was former Edwards aide Andrew Young who’d “milked” the heiress Bunny Mellon on his own, and pocketed most of the proceeds. Then, the defense says, he went to another Edwards friend and donor, Fred Baron, for reimbursement for money he claimed to have spent on Hunter.

Edwards faces up to 30 years in prison and $1.5 million in fines on six counts – federal charges that include conspiracy, false statements and taking illegal campaign contributions.

Though sin also requires intent, defense attorney Allison Van Laningham referred to her client as “a man who has committed many sins but no crimes,’’ and who was trying to avoid humiliation rather than scheming to defraud the public.

Just as the defense acknowledged having an unlikable cur of a client, the prosecution copped to a seriously flawed star witness in Young, who like Edwards is an accomplished liar, at one point claiming Edwards’ daughter with Hunter as his own.

“Since he can no longer make money being for John Edwards,’’ Van Laningham said, “he wants to make money being against him’’ and published a tell-all book.

When Young took the stand Monday afternoon, he spoke at length about how he’d wormed his way into John Edwards’s life – driving him everywhere, picking up his dry cleaning, changing the lightbulbs in his home, doing anything at all that needed doing, really.

And in the end, the case will turn on which worm turned on the other, and which one we believe.

Melinda Henneberger is a Post political writer, and anchors the paper’s ‘She the People’ blog. Follow her on Twitter at @MelindaDC.