“I felt a shudder down my spine yesterday watching Donald Trump’s [fusillade] against the press. This is not a moment to be trifled with. It wasn’t his first tirade and it won’t be his last,” Rather wrote in a Facebook (FB) post that came out swinging in the wake of the presumptive GOP presidential nominee’s Tuesday press conference. At the event, Trump said “the press should be ashamed of themselves” before describing the political press as “disgusting” and calling one ABC reporter “a sleaze.”

Trump’s anti-press sentiments rankled Rather, the former nightly news anchor for CBS’ (CBS) Evening News. In his Facebook post, Rather warns that Trump’s attacks on the media carry “the drumbeats of threatening violence” and he claims that the issue is not purely political, writing that it is “about protecting our most cherished principles.”

Here is Rather’s seven-paragraph Facebook post in full:

I felt a shudder down my spine yesterday watching Donald Trump’s fusilade against the press. This is not a moment to be trifled with. It wasn’t his first tirade and it won’t be his last.

I was reminded of my college journalism professor, the late Hugh Cunningham, who would exhort his young charges in a thundering voice to “never let them scare you.” It was his most important lesson. One of Edward R. Murrow’s favorite words was “steady.” That also bears repeating today.

This is a dirty, nasty election. And it is only going to get worse. The reporters in the trenches need no lecture from me. They are walking through daily minefields, bracing themselves against winds of discontent whose effects no one can predict.

I know what it is like to sit in those seats and feel the scorn and even wrath of politicians of all political persuasions. Attacking the press for unfair coverage has long been a bipartisan pursuit. Sometimes it works. I am happy to say that more often it doesn’t. But Trump’s brand of vituperation is particularly personal and vicious. It carries with it the drumbeats of threatening violence. It cannot be left unanswered.

This is not about politics or policy. It’s about protecting our most cherished principles. The relationship between the press and the powerful they cover is by its very definition confrontational. That is how the Founding Fathers envisioned it, with noble clauses of protection enshrined in our Constitution.

Good journalism–the kind that matters–requires reporters who won’t back up, back down, back away or turn around when faced with efforts to intimidate them. It also requires owners and other bosses with guts, who stand by and for their reporters when the heat is on.

I still believe the pen is mightier than the sword. And in these conflicted and troubled times, we should reward the bravery of the men and women not afraid to ask the hard questions of everyone in power. Our nation’s future depends on it.

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Trump’s most recent tirade came in response to journalists pressing him to reveal the specifics of his claimed contributions to U.S. veterans groups, but it was far from the first time Trump has butted heads with the news media in the current election cycle. Despite the prevailing opinion that the media has contributed to Trump’s surprising ascendancy, the billionaire and former reality TV star has seemed to wage something of a war on the press, going so far as to say he would “open up our libel laws” as president while describing journalists as “dishonest” and “scum.”