Trump's Twitter outburst - in which he describes the journalist "bleeding badly from a facelift" - appears to have been triggered by Brzezinski's sharp comments during the morning show she co-hosts with Joe Scarborough on the left-leaning MSNBC network.
"I heard poorly rated @Morning_Joe speaks badly of me (don't watch anymore)," wrote Trump, an avid cable news watcher.
"Then how come low I.Q. Crazy Mika, along with Psycho Joe, came to Mar-a-Lago 3 nights in a row around New Year's Eve, and insisted on joining me. She was bleeding badly from a face-lift. I said no!" he tweeted.
In recent days Trump has escalated his attacks on US media outlets -- which he accuses of peddling "fake news" about him -- singling out CNN over the retraction of a story about his administration's supposed links with Russia, but also assailing rival cable networks, the New York Times and the Washington Post.
But the vulgarity of his latest assault triggered a torrent of criticism including within his own party.
"Inappropriate. Undignified. Unpresidential," tweeted Jeb Bush, Trump's rival for the White House nomination last year, while the top Republican on Capitol Hill, House Speaker Paul Ryan, conceded the president's comments were "not appropriate."
"What we're trying to do around here is improve the tone, the civility of the debate. And this obviously doesn't help do that," Ryan told a news conference.
'Out of his mind'
Brzezinski - the daughter of the late Zbigniew Brzezinski, national security advisor to Jimmy Carter -- herself appeared uncowed. She responded with a post of her own, a photo of a young child reaching for Cheerios cereal.
"Made for little hands," the caption read - a schoolyard taunt that has been leveled against the image-conscious Trump since the days of his candidacy.
In her Thursday show, Brzezinski suggested the president was psychologically unhinged.
"Everybody in Washington, in the administration, needs to look at this from... a point of view where they're not lobotomized because you're sitting in there and you're so scared of him, and you think you need to suck up to the president."
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If a boss at NBC, MSNBC's parent company, "started tweeting wildly about people's appearances, bullying people, talking about people in the competition, lying every day, undermining his managers, throwing them under the (bus) -- that person would be thrown out.
"There would be concerns that the person who runs the company is out of his mind."
Early in the 2016 campaign the "Morning Joe" hosts were accused of being too friendly with Trump, a frequent guest on air.
The relationship however soured last August, when Trump tweeted: "Some day, when things calm down, I'll tell the real story of @JoeNBC and his very insecure long-time girlfriend, @morningmika. Two clowns!"
Brzezinski, 50, is engaged to marry her co-host Scarborough, 54, a Republican former congressman.
Twitter ablaze
MSNBC sprang to Brzezinski's defense.
"It's a sad day for America when the president spends his time bullying, lying and spewing petty personal attacks instead of doing his job," the network's public relations office wrote.
Scarborough likewise rallied behind his fiancee, re-tweeting a post from NBC spokesman Mark Kornblau that read: "Never imagined a day when I would think to myself, 'it is beneath my dignity to respond to the President of the United States.'"
White House spokeswoman Sarah Huckabee Sanders was quick to defend her boss.
"I don't think that the president's ever been someone who gets attacked and doesn't push back," Huckabee Sanders told Fox News.
"There have been an outrageous number of personal attacks, not just to him but to frankly everyone around him ... This is a president who fights fire with fire and certainly will not be allowed to be bullied by liberal media or liberal elites in Hollywood or anywhere else."
But at least half a dozen Republican lawmakers judged the president had crossed a line.
"Please just stop," wrote Senator Ben Sasse on Twitter. "This isn't normal and it's beneath the dignity of your office."
"This is not okay," agreed Kansas lawmaker Lynn Jenkins.
The Trump-Brzezinski feud lit up the internet, where critics were reminded of Trump's notorious attack last August on Fox reporter Megyn Kelly, who he said had "blood coming out of her wherever" after she challenged him in a campaign debate.
"Harry Potter" author and noted Trump critic J.K. Rowling took inspiration from a revered past president in writing: "'Nearly all men can stand adversity, but if you want to test a man's character, give him power.' Abraham Lincoln."
