Former vice presidential nominee John Edwards was indicted Friday on charges of using illegal campaign donations to conceal his mistress from voters, a stunning fall from grace for a politician once considered a serious contender for the White House.

The indictment triggered immediate criticism from a range of campaign finance and legal experts, who said the government’s case is unprecedented and appears weak. They questioned whether it would survive the fierce defense being mounted by Edwards’s lawyers, who say his actions did not constitute a crime.

“It’s not illegal to be a pig,” said Brett Kappel, a Washington campaign finance expert who has worked for Republicans and Democrats. “Is what Edwards did slimy? Absolutely. Everyone will agree it was reprehensible. But it’s not a crime.”

Some good-government groups welcomed the Justice Department’s decision to charge Edwards in a six-count indictment that says he helped solicit what prosecutors called nearly $1 million in illegal contributions from political donors. Prosecutors said his motive was to hide his mistress, Rielle Hunter, and her pregnancy from the public and media.

The payments, which covered Hunter’s living, medical and other expenses, are at the heart of the dispute. Prosecutors say that they were campaign contributions because they were meant to prevent Edwards’s 2008 presidential bid from collapsing if the affair was discovered. They say the contributions also exceeded legal limits.

Defense lawyers are expected to argue that the funds never went to the campaign and were intended as gifts from two Edwards supporters. The disclosure of Edwards’s affair with Hunter, and his initial denials that he fathered a child with her, wound up destroying his once-promising political career.

The indictment, returned in federal court in North Carolina, retells the familiar and sordid details of the Edwards saga, arguing that they were part of an elaborate criminal conspiracy. It says Edwards and an aide plotted to solicit the contributions, some of which went to house Hunter at fancy hotels.

One of the donors, Rachel “Bunny” Mellon, who is 100, is quoted as offering to help pay some of Edwards’s bills as “a way to help our friend without government restrictions.”

“We will not permit candidates for high office to abuse their special ability to access the coffers of their political supporters to circumvent our election laws,” said Lanny A. Breuer, assistant attorney general for the Justice Department’s Criminal Division. “Our campaign finance system is designed to preserve the integrity of democratic elections . . . and we will vigorously pursue abuses of the kind alleged today.”

The hotly contested case will pose another challenge for Justice’s Public Integrity Section, which has been recovering from the collapse of its 2008 case against the late Sen. Ted Stevens (R-Alaska). Stevens was convicted on corruption charges, but prosecutorial wrongdoing led Attorney General Eric H. Holder Jr. to ask the judge to drop the case in 2009. The section has won eight trials nationwide since January.

Edwards, who appeared in federal court in Winston-Salem, N.C., on Friday and was released on his own recognizance, acknowledged that he has “done wrong, and I take full responsibility for having done wrong. And I will regret for the rest of my life the pain and the harm that I have caused to others.’’

But the former candidate and onetime legendary North Carolina trial lawyer insisted that he did nothing illegal and said: “I never, ever thought I was breaking the law.”

Lawyers for Edwards said they will vigorously fight the charges, which include conspiracy, false statements and illegal campaign contributions. “This is an unprecedented prosecution,’’ said Gregory Craig, Edwards’s lawyer.

Donors’ contributions

The indictment says Edwards conspired to receive and cover up illegal contributions of about $725,000 from an unidentified “Person C” and more than $200,000 from “Person D.’’ Person C is known to be Mellon, the daughter of a wealthy industrialist, and Person D is known to be Fred Baron, Edwards’s campaign finance chairman in 2008. Baron, a longtime Democratic party activist, died in October 2008.

Prosecutors said that the payments violated the $2,300 limit on contributions from individuals and that Edwards concealed them from the Federal Election Commission.

The indictment appears to try to show that Mellon and Baron knew they were doing something illicit. It cites Mellon’s note to “Person A” — known to be former Edwards aide Andrew Young — offering to help with Edwards’s expenses after he was criticized in the media for receiving a $400 haircut.

It quotes the person known to be Baron as writing a note to Young in an envelope stuffed with $1,000 cash: “Old Chinese saying: use cash, not credit cards.’’

Baron told the Associated Press two months before his death that Edwards had no knowledge of what he did. “I did it as a friend,’’ he said.

Legal experts said prosecutors will have difficulty showing that Mellon and Baron intended the funds as campaign contributions. “You have to show criminal intent. That’s a stretch,’’ said Stanley Brand, a Democratic campaign finance law expert. “One of them is dead, and the other is 100 years old.’’

He added, “I can’t remember any case where this type of criminal allegation was made in a campaign context.’’

Credibility a concern

Other experts said the government’s case will be hurt by its reliance on Young, who has admitted that he lied in claiming paternity of Hunter’s child and has been criticized over his handling of a purported sex tape of the two, which he had said he possessed.

Lawyers for Edwards “will seriously damage Young’s credibility,’’ said Jason Torchinsky, a Republican election lawyer, who said the government has “a decent case but definitely not a slam-dunk.’’

The charges came after the collapse of plea-bargain negotiations with the Justice Department. Sources familiar with the talks have said that Edwards, a single father since the death in December of his estranged wife, Elizabeth, had wanted to put the matter behind him.

A source close to the case would not specify why the talks collapsed but said, “Both parties moved heaven and earth to try to make it possible to resolve this short of a trial, and it just didn’t work.’’

Hunter was a videographer for Edwards’s 2008 presidential campaign. After his initial denials, Edwards admitted in January 2010 to being the father of Hunter’s daughter, Quinn. Shortly afterward, he separated from his wife, a prominent Democratic activist.

In 2004, Edwards unsuccessfully sought the Democratic presidential nomination, then ran for vice president with the eventual nominee, Sen. John F. Kerry (Mass.). Edwards did not seek reelection to the Senate seat from North Carolina that he had won in 1998. He suspended his second presidential campaign in January 2008 after poor showings in the initial primaries.

Meredith McGehee, policy director for the Campaign Legal Center, called the allegations against Edwards “deeply disturbing” and praised the Justice Department for bringing the case.

“To let these actions pass without bringing them to a court of law would be to set a dangerous precedent,’’ she said.

Staff writer William Branigin contributed to this report.