A longtime analyst for Fox News is leaving the network, saying that he could not “in good conscience” remain with an organization that, he argued, “is now wittingly harming our system of government for profit.”

In a searing farewell note sent to colleagues on Tuesday, Ralph Peters, a Fox News strategic analyst and a retired lieutenant colonel in the United States Army, castigated the network for its coverage of President Trump and the rhetoric of its prime-time hosts.

“In my view, Fox has degenerated from providing a legitimate and much-needed outlet for conservative voices to a mere propaganda machine for a destructive and ethically ruinous administration,” Colonel Peters wrote in his message, a copy of which was obtained by The New York Times.

“Over my decade with Fox, I long was proud of the association,” he added. “Now I am ashamed.”

Without citing them by name, Colonel Peters, 65, wrote that Fox News’s prime-time anchors “dismiss facts and empirical reality to launch profoundly dishonest assaults on the F.B.I., the Justice Department, the courts, the intelligence community (in which I served) and, not least, a model public servant and genuine war hero such as Robert Mueller.”