Scarborough's brother, George, an outspoken Trump supporter, lashed out at the president for pushing a "bold lie" from more than 20 years ago that "has put a huge emotional drain on my family".
"Looks like I picked a good day to stop responding to Trump's weird tweets", Scarborough wrote to his 1.96 million followers. "He is not well".
The president was referring to the mysterious death of one of Scarborough's young female staffers, Lori Klausutis, in 2001 while he served as a Florida congressman. Trump specifically went after MSNBC host and former U.S. Rep. Joe Scarborough, who previously served as a four-term Congressman: "And will they terminate low ratings Joe Scarborough based on the "unsolved mystery" that took place in Florida years ago?" And we are facing a showdown with a nuclear power and you have somebody inside the White House that the New York Daily News says is mentally unfit. "Is that when it's proper to bring this up in polite society?"
"If this is not what the 25th Amendment was drafted for..."
"You have somebody inside the White House that the New York Daily News says is mentally unfit", Mr Scarborough said.
SCARBOROUGH: When are we supposed to say this, after the first nuclear missile goes?
This is just the latest in a long feud between Trump and the "Morning Joe" duo.
Scarborough ended his rant by saying Republicans want Vice President Mike Pence to be president. "He is not well", responded Scarborough on Twitter on Wednesday morning. The intern, who was found dead in an office, collapsed of an un-diagnosed heart condition and hit her head on the side of a desk, causing a fatal blood clot, the local medical examiner determined at the time.
Before the 2016 presidential election, Scarborough received criticism for being too close to Trump, but has since shifted his stance. Bob Corker (R-Tenn.) said "the president has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful".