(CNN) Former national security adviser John Bolton defended his ex-colleague, former White House chief of staff John Kelly, after President Donald Trump belittled Kelly for comments backing an impeachment witness.

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

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. Conservatives especially have a responsibility to reject baseless attacks upon him. — John Bolton (@AmbJohnBolton) February 13, 2020

Bolton's comments continue a trend of the two men joining forces of late against Trump's attacks, despite their rocky relationship when they served together in the West Wing.

The Atlantic reported that, during remarks on Wednesday, Kelly asserted that Lt. Col. Alexander Vindman, a key witness in Trump's impeachment inquiry, had been right to raise concerns about the President's July call with Ukraine's President and that he had told the truth during his testimony.

The comments attracted the ire of Trump, who responded with a pair of tweets disparaging his former chief of staff, a retired Marine Corps general, on Thursday morning.

Read More