ABC News Chief Anchor George Stephanopoulos, who for 15 years expertly navigated his re-invention from tough Clinton operative to respected television journalist, has finally—and perhaps inevitably—slipped up.

His mistake, by any definition, is a beaut—no doubt prompting a blush of shame and putting the network news division on the defensive.

It turns out that the 54-year-old Stephanopoulos—who served as a top aide in Bill Clinton’s 1992 presidential campaign and first White House term before leaving to teach, write an acclaimed memoir and join ABC—failed to disclose to his ABC News bosses $75,000 in contributions he made to the Clinton Foundation.

Worse, he didn’t tell viewers, keeping silent about the potential conflict of interest even as he conducted a contentious interview April 26 on his Sunday panel show, This Week With George Stephanopoulos, with Clinton Foundation critic Peter Schweizer, author of Clinton Cash.

In an interview with Politico media columnist Dylan Byers, who broke the story Thursday morning, Stephanopoulos apologized profusely to his viewers and ABC News colleagues for his misstep, announced that he would no longer moderate the scheduled ABC-sponsored Republican primary debate next February, promised to address the embarrassing issue on Sunday’s program, and expressed regret that he had given the Clinton Foundation money in the first place.

ABC initially said he’d donated a total of $50,000, but Stephanopoulos later corrected that figure, saying he’d written $25,000 checks in each of 2012, 2013, and 2014.

"In retrospect, I probably shouldn't have, even though I did it for the best reasons," Stephanopoulos said about his donations.

In a statement earlier Thursday, he had said: "I made charitable donations to the Foundation in support of the work they’re doing on global AIDS prevention and deforestation, causes I care about deeply. I thought that my contributions were a matter of public record. However, in hindsight, I should have taken the extra step of personally disclosing my donations to my employer and to the viewers on air during the recent news stories about the Foundation. I apologize."

Schweizer, meanwhile, was unimpressed and accused Stephanopoulos of a "massive breach of ethical standards," writing in an email to Bloomberg Politics that he was "really quite stunned by this...He fairly noted [during the This Week appearance] my four months working as a speechwriter for George W. Bush. But he didn't disclose this?”

Stephanopoulos—who is also the network’s chief political correspondent and cohost of Good Morning America—also told Byers that he will stay away from the February debate, which he had been scheduled to moderate.

"I won't moderate that debate,” he said. “I think I've shown that I can moderate debates fairly. That said, I know there have been questions made about moderating debates this year. I want to be sure I don't deprive moderators or viewers of a good debate."

That assessment is not universally shared, however, and shortly after the donation was revealed, presidential candidate Rand Paul, the Republican junior senator from Kentucky, declared that Stephanopoulos should be barred from moderating any GOP debates next year.

The communications director for Republican Sen. Mike Lee of Utah announced on Twitter: “I’m not letting my boss go on @ABC until @GStephanopoulos recuses himself from all 2016 coverage.”

After that tweet from his aide, Sen. Lee said in a statement to The Daily Beast: “I will ultimately make the final decision about each interview request we receive. But I am very concerned about how George’s donations to the Clintons calls into question his journalistic objectivity. You don’t give $75,000 to an organization and then forget to mention it when you interview a person who is questioning that organization. This fact will weigh heavily as I consider future media requests from ABC News."

It was unclear if other Republican office-holders would also protest, but Stephanopoulos—who didn’t respond to an email seeking comment—told Byers he will continue to cover the campaign.

“It’s a mistake, and it’s a dumb one, but it’s not a criminal offense,” said Columbia University Journalism School professor Dick Wald, a former vice president and “ethics czar” of ABC News. “Other people have done other dumb things.”

Wald told The Daily Beast that if Stephanopoulos had disclosed his contributions to viewers, there would have been nothing wrong with his doing the Schweizer interview.

“Audiences aren’t dumb,” Wald said. “They are perfectly capable of making decisions about whether or not this was a reasonable series of questions and answers.”

Asked if it wouldn’t have been better if Stephanopoulos had recused himself and delegated the interview to another correspondent, Wald answered: “They could have switched, but that’s not necessary. It might have been a smart thing to do in retrospect.”

An ABC News statement Thursday morning reiterated Stephanopoulos’s explanation, and noted that “he’s admitted to an honest mistake and apologized for that omission. We stand behind him.”

The spokesperson declined to address the question of whether ABC News President James Goldston, had he known of the $75,000 donation, would have permitted Stephanopoulos to conduct the Schweizer interview.

“I don’t want to talk about a hypothetical situation,” the spokesperson told The Daily Beast. “He apologized. He recognizes his mistake. We accept that apology. We believe it was an honest mistake.”

While on-air personalities at other news outlets have been punished for similar mistakes—MSNBC suspended hosts Keith Olbermann and Joe Scarborough for making political contributions without first obtaining permission—Stephanopoulos will face no disciplinary action, the ABC News spokesperson said.

Wald, meanwhile, said Stephanopoulos has proven to viewers, over many years, that he can be skeptical and dispassionate when reporting on his former employers. Indeed, in 1998, after Stephanopoulos was among the first political analysts to suggest on the air that the then-exploding Monica Lewinsky scandal could lead to impeachment, the Clintons and their loyalists were said to be enraged.

“George Step-on-top-of-us,” Clinton pal Harry Thomason dubbed him at the time.

“Nobody is under the false impression that George and the Clintons have never met,” Wald said. “Everybody knows that George was an important political operator for them. But he has demonstrated over the years—now quite a large number of years—that he is independent of the politics of the Clintons.”

While Clinton Cash’s sensational charges have received respectful attention from the The New York Times and other media outlets, Hillary Clinton’s presidential campaign has worked hard to debunk Schweizer’s thesis that the Clinton state department and former president Clinton did favors for business people who donated generously to the family foundation.

“We’ve done investigative work here at ABC News, found no proof of any kind of direct action,” Stephanopoulos told Schweizer on the air. “An independent government ethics expert, Bill Allison, of the Sunlight Foundation, wrote this. He said, ‘There’s no smoking gun, no evidence that she changed the policy based on donations to the foundation.’ No smoking gun.”

Stephanpoulos also pushed back on Schweizer’s assertion that there should be a criminal investigation of how Hillary Clinton’s actions at the State Department, which endorsed a lucrative uranium deal that benefited foundation contributors, might be connected to that largesse.

“That—that is an issue for them, but it's not a criminal—it's nothing that would warrant a criminal investigation,” Stephanopoulos declared, according to the ABC News transcript.