Brian Karem — the executive editor of the Montgomery County and Prince George's Sentinel newspapers, which serve the Maryland suburbs — accused Sanders of "inflammatory" comments to the press.
"We're here to ask you questions," Karem said. "You're here to provide the answers, and what you just did is inflammatory to people all over the country who look at it and say, 'See, once again, the president's right and everybody else out here is fake media,' and everybody in this room is only trying to do their job."
His passionate outburst came after the White House spokeswoman repeatedly assailed the news media for what she said was unfair coverage of the Trump administration. She cited journalists' use of unnamed sources and continued coverage of the "Russia/Trump hoax."
"If we make the slightest mistake, the slightest word is off, it is just an absolute tirade from a lot of people in this room," Sanders said, "but news outlets get to go on, day after day, and cite unnamed sources, use stories without sources."
But it was when Sanders referenced the resignation of three CNN employees following a retracted story on Russia that Karem grew frustrated.
"Come on. You're inflaming everybody right here, right now with those words. This administration has done that as well," he said, referring to the use of anonymous sources.
"Why in the name of heavens — any one of us are replaceable and any one of us, if we don't get it right, the audience has the opportunity to turn the channel or not read us," he said. "You have been elected to serve for four years at least. There's no option other than that."
Sanders said she disagreed completely with Karem's assertions.
"I think if anything has been inflamed it's the dishonesty that often takes place by the news media, and I think it is outrageous for you to accuse me of inflaming a story when I was simply trying to respond to his question," she said.
Karem, who also writes a regular political column for Playboy magazine, told BuzzFeed News he spoke up during the briefing because he became frustrated "over the tainting all of us with the brush of 'fake news' and total disrespect for what we do."
"When we make mistakes, we issue corrections, people get fired, there are consequences. I've yet to see any consequences for this administration," he said.
The exchange quickly lit up social media, with many praising Karem for his highly unusual televised confrontation with a White House official.
Karem also quickly gained tens of thousands of new Twitter followers.
The integrity of the news media is a subject close to Karem's heart, he said, because he once spent time in prison for refusing to identify a confidential source.
While working at a San Antonio television station in 1989, Karem spoke by phone with a murder suspect in prison, but refused to hand over his notes or identify who had set up the call. He was jailed for six months in 1990, but served only two weeks after Karem's source was uncovered by others and she allowed Karem to identify her.
"Journalists will applaud Karem's stand for the principle that reporters should not be compelled to reveal confidential sources," read a 1992 Publishers Weekly review of the book he wrote about his jailing.
"My time in jail taught me that while I don't take myself too seriously, I take what we do seriously," he said. "There are journalists who have given their life for the cause, who have been jailed or beaten for the cause."
Sanders did not immediately return a request for comment.
Karem said he had respect for, and gets along with, Sanders and press secretary Sean Spicer.
"But I wish the president of the United States and his staff had a little bit more respect for what we do," he said.