Fox News is a "propaganda machine." It is "wittingly harming our system of government for profit."

Fox critics -- mostly on the left -- have been making arguments like these for years. There have been books and documentaries dedicated to it.

What's astonishing is that now these words are a Fox News contributor's.

Ralph Peters, a longtime Fox News military analyst, said Tuesday that he is leaving the network. And he's leaving in the most dramatic way possible: By sending to colleagues a blistering critique of the network as pro-Trump "propaganda."

Peters wrote in a letter to a handful of colleagues that Fox is "assaulting our constitutional order and the rule of law." He called it "a destructive and ethically ruinous administration."

Democrats -- and a few Republicans -- cheered. Bruce Bartlett, a writer and historian who worked in the Reagan and H.W. Bush administrations, said, "I'm guessing that he made a calculation that Trump is going down and Fox may go down with him."

Bartlett is one of the rare Republicans who has been fiercely critical of Fox News.

He praised Peters as "someone on the right with integrity."

"Peters is the conservative establishment and as far as I can see has no policy disagreements with the GOP or the conservative movement," Bartlett told CNN in an email. "His concern is simply that Fox isn't a responsible media outlet. It is propaganda (his own words). I think that makes his disagreement with Fox far more telling and interesting."

Some past sound bites from Peters' career were resurfaced in the wake of the letter. In one 2015 Fox Business segment, for instance, Peters called then-president Barack Obama a "total p***y."

After BuzzFeed published his scorching letter on Tuesday, some Fox skeptics in the media praised him for speaking out.

But Angelo Carusone, the president of the anti-Fox media monitoring group Media Matters, had a very different reaction.

"Peters deserves no accolades here," Carusone said. "For more than a decade, he was reliable and unwavering in advancing Fox News' agenda."

"That said," Carusone added, "it's notable that a Roger Ailes O.G. is saying that Fox News has gone from being a partisan political operation to a propaganda machine and that Fox News is now actively working to undermine the rule of law. It should alarm everyone that the current iteration of Fox News, in particular its prime time lineup, is so far out there that it's out of sync with someone as extreme as Peters."

Progressive filmmaker Robert Greenwald, who interviewed former Fox employees for the 2004 documentary "Outfoxed," said he agreed with Peters that "propaganda machine" is "an apt and accurate term" for the network.

"The blind and ruthless kissing the ring of anything Trump has gone beyond all their previous propaganda work," Greenwald said.

Fox News responded to Peters' remarks in a statement to CNN: "Ralph Peters is entitled to his opinion despite the fact that he's choosing to use it as a weapon in order to gain attention. We are extremely proud of our top-rated primetime hosts and all of our opinion programing."