The president went after the hosts, who have criticized Trump, in a series of tweets.
I heard poorly rated @Morning_Joe speaks badly of me (don't watch anymore). Then how come low I.Q. Crazy Mika, along with Psycho Joe, came..— Donald J. Trump (@realDonaldTrump) June 29, 2017
...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!— Donald J. Trump (@realDonaldTrump) June 29, 2017
Brzezinski responded on Twitter by posting a photograph of a Cheerios box that has the phrase "made for little hands."
Critics looking to get under the president's skin have long suggested that his hands appear smaller than usual for his frame.
Republican Senators Lindsey Graham, R-S.C., and Ben Sasse, R-Neb., both frequent Trump critics, assailed the president on Twitter.
Mr. President, your tweet was beneath the office and represents what is wrong with American politics, not the greatness of America.— Lindsey Graham (@LindseyGrahamSC) June 29, 2017
Please just stop. This isn't normal and it's beneath the dignity of your office.— Ben Sasse (@BenSasse) June 29, 2017
Meghan McCain, a media personality and the eldest daughter of Sen. John McCain, also criticized Trump's tweets.
I do not think making fun of a woman's looks is acceptable. I get it every day of my life. I think that tweet is cruel - and unpresidential— Meghan McCain (@MeghanMcCain) June 29, 2017
She wrote in another tweet, "What does Ivanka think of this?"
What does Ivanka think of this? https://t.co/4AenXJMA7t— Meghan McCain (@MeghanMcCain) June 29, 2017
The White House is defending Trump's tweets. Deputy press secretary Sarah Huckabee Sanders told Fox News that the president has been subject to "outrageous" personal attacks on "Morning Joe."
"This is a president who fights fire with fire and certainly will not be allowed to be bullied by the liberal media and the liberal elites within the media or in Hollywood or anywhere else," she said.
First Lady Melania Trump, who has said she wants to run an effort to combat cyber-bullying, also defended her husband's conduct.
"As the first lady has stated publicly in the past, when her husband gets attacked, he will punch back 10 times harder," said her spokeswoman, Stephanie Grisham.
Still, the communications team for MSNBC didn't hold back in its response.
"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 channel tweeted.
About two hours before the president's tweets, Brzezinski said on the show that "it's not normal behavior" for any leader to be tweeting about people's appearances, bullying, lying, undermining managers and throwing people under the bus.
Saying that if any business executive behaved the way Trump does, "There would be concern that perhaps the person who runs the company is out of his mind."
On their Wednesday show, Brzezinski and Scarborough roundly mocked Trump for displaying in several of his golf resorts a fake Time Magazine cover featuring himself.
"That's needy," Brzezinski said on the show.
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About 15 minutes before the president himself tweeted, White House social media director Dan Scavino similarly attacked the hosts.
"#DumbAsARockMika and lover #JealousJoe are lost, confused & saddened since POTUS realDonaldTrump stopped returning their calls! Unhinged," Scavino wrote on his personal account.
Trump was correct that the MSNBC hosts spent time at the president's Florida resort, a visit that Scarborough said was to arrange a Trump interview.
The hosts confirmed they are engaged to be married earlier this month.
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The Associated Press and ABC News contributed to this report.