Former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE on Thursday came to the defense of former chief of staff John Kelly John Francis KellyMORE after President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE lashed out over a series of criticisms Kelly made during an appearance at a New Jersey university.

"John Kelly is an honorable man. John and I have disagreed at times, as is commonplace at senior government levels, but he has always served his country faithfully," Bolton tweeted. "Conservatives especially have a responsibility to reject baseless attacks upon him."

Bolton and Kelly overlapped during their time at the White House, and their relationship was not always cordial. The two reportedly engaged in a shouting match in October 2018 during a heated argument about immigration.

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But they have defended each other at points after they both came under attack from Trump.

Trump ripped Kelly earlier Thursday in response to the former chief of staff's comments during a lecture series event at Drew University. Kelly defended impeachment witness Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, said the media is "not the enemy of the people," and described migrants as "overwhelmingly good people."

"When I terminated John Kelly, which I couldn’t do fast enough, he knew full well that he was way over his head. Being Chief of Staff just wasn’t for him," Trump tweeted. "He came in with a bang, went out with a whimper, but like so many X’s, he misses the action & just can’t keep his mouth shut, which he actually has a military and legal obligation to do."

Kelly, who also served as the Homeland Security secretary, stepped down as chief of staff in January 2019.

Trump has also repeatedly gone after Bolton as excerpts of his forthcoming book have leaked out. The president accused Bolton of being a warmonger, claimed he "begged" for a job and asserted his former adviser clashed with other White House officials.

Kelly, however, offered support for Bolton after Trump denied allegations levied in the book.

“If John Bolton says that in the book, I believe John Bolton,” he said.