Former CIA Director John BrennanJohn Owen BrennanOvernight Defense: Capitol Police may ask National Guard to stay | Biden's Pentagon policy nominee faces criticism | Naval Academy midshipmen moved to hotels Republicans blast Pentagon policy nominee over tweets, Iran nuclear deal Online and frighteningly real: 'A Taste of Armageddon' MORE on Thursday explained his intense criticism of President TrumpDonald TrumpUS, South Korea reach agreement on cost-sharing for troops Graham: Trump can make GOP bigger, stronger, or he 'could destroy it' Biden nominates female generals whose promotions were reportedly delayed under Trump MORE, saying that he is hoping to shake "some sense" into the president's aides and Republican lawmakers.

"When I see the types of things that Mr. Trump is doing, yes, I will speak out," Brennan said on MSNBC. "I'm hoping to be able to shake some — quite frankly — some sense into the people around Mr. Trump and those on the Hill."

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"The more that they tolerate and they make excuses for his behavior and his performance, the more they're enabling him, the more they're encouraging him to do these types of things," he added.

Brennan, who served as CIA director under former President Obama, has been sharply critical of Trump since he took office last year.

He ramped up his criticism this week after Trump's performance in a controversial news conference with Russian President Vladimir Putin, in which Trump appeared to side with Putin's denials over the U.S. intelligence community's assessment that Moscow interfered in the 2016 presidential election.

After that news conference, Brennan hit Trump on Twitter, calling his comments "imbecilic" and "nothing short of treasonous."

That prompted a blistering response from Trump, who called Brennan "a very bad person" during an interview with Fox News host Tucker Carlson that aired Tuesday.

"I think Brennan is a very bad guy, and if you look at it, a lot of things happened under his watch," Trump said. "I think he's a very bad person."

Brennan responded to that attack on Thursday, saying on MSNBC that he is simply "trying to ring the alarm bell" on issues that he believes are cause for concern.

"Some of my tweets I will acknowledge are rather shrill if not strident," he said. "Maybe I'm just an intelligence officer and I'm trying to ring the bell, because I see all the warning signs here, especially when it comes to national security."