(CNN) A defiant President Donald Trump resumed his attacks on MSNBC's "Morning Joe" co-hosts Joe Scarborough and Mika Brzezinski on Saturday morning, calling Scarborough "crazy" and Brzezinski "dumb as a rock," despite days of bipartisan criticism over his initial attacks on the pair earlier this week.

"Crazy Joe Scarborough and dumb as a rock Mika are not bad people, but their low rated show is dominated by their NBC bosses. Too bad!" Trump tweeted Saturday morning.

Crazy Joe Scarborough and dumb as a rock Mika are not bad people, but their low rated show is dominated by their NBC bosses. Too bad!

Republican and Democratic lawmakers rebuked Trump's attacks on the hosts Thursday after he slammed Brzezinski's intellect, questioned her sanity and mocked her appearance.

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..

...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!

"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... 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!" Trump wrote in a series of tweets.

On Friday, Scarborough and Brzezinski responded to Trump's attacks with an opinion piece in The Washington Post titled "Donald Trump is not well."

"America's leaders and allies are asking themselves yet again whether this man is fit to be president," they wrote. "We have our doubts, but we are both certain that the man is not mentally equipped to continue watching our show, 'Morning Joe.'"

During their Friday "Morning Joe" broadcast, Scarborough and Brzezinski also accused Trump and his White House of using the possibility of a hit piece in the National Enquirer to threaten them and change their news coverage, to which Trump responded on Twitter by calling that accusation "FAKE NEWS. He called me to stop a National Enquirer article. I said no! Bad show."

Watched low rated @Morning_Joe for first time in long time. FAKE NEWS. He called me to stop a National Enquirer article. I said no! Bad show — Donald J. Trump (@realDonaldTrump) June 30, 2017

But Scarborough said he has proof of White House threats earlier this year; he replied to Trump by tweeting, "Yet another lie. I have texts from your top aides and phone records."

Yet another lie. I have texts from your top aides and phone records. Also, those records show I haven't spoken with you in many months. https://t.co/TZWiElo6Gs — Joe Scarborough (@JoeNBC) June 30, 2017

A White House official suggested to CNN that nothing untoward occurred, saying Scarborough called Trump's son-in-law, Jared Kushner, about the Enquirer story before it was published. The official said Kushner told Scarborough to call the President. The official denied there was ever any offer from Kushner of a quid pro quo -- in other words, softer coverage in exchange for spiking the Enquirer story.

The White House did not return a request for comment on Trump's latest tweet on Saturday.

Deputy White House press secretary Sarah Huckabee Sanders defended Trump's initial attack at Thursday's White House press briefing, saying, "Look, the American people elected a fighter. They didn't elect somebody to sit back and do nothing." She added that Trump "fights fire with fire."

Sanders also said on Fox News that Trump was responding to liberal bullying when he tweeted about the MSNBC host.

"I don't think that the President's ever been someone who gets attacked and doesn't push back," she told the network Thursday. "There have been an outrageous number of personal attacks not just to him but people around him."

First lady Melania Trump also stood by her husband's remarks about the hosts.

"As the first lady has stated publicly in the past, when her husband gets attacked, he will punch back 10 times harder," the first lady's communications director, Stephanie Grisham, said in a statement to CNN on Thursday.

When asked about Trump's initial tweets during a House Republican news conference Thursday, House Speaker Paul Ryan responded, "Obviously I don't see that as an appropriate comment."

He added: "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."

Other Republican lawmakers reacted by making public pleas to the President to stop the personal attacks.

Please just stop. This isn't normal and it's beneath the dignity of your office. — Ben Sasse (@BenSasse) June 29, 2017

"Please just stop. This isn't normal and it's beneath the dignity of your office," GOP Sen. Ben Sasse of Nebraska tweeted shortly after Trump launched into a tweetstorm about the show.

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

South Carolina Republican Sen. Lindsey Graham also tweeted Thursday: "Mr. President, your tweet was beneath the office and represents what is wrong with American politics, not the greatness of America."