Rep. Ruben Gallego lashed out against Republican nominee Donald Trump, calling his character into question.

Citing Trump's attacks on the parents of a slain U.S. Muslim soldier, the Arizona Democrat told CNN's Carol Costello on Tuesday that Trump "is unstable, he's unfit to be a presidential candidate."

Costello was caught off guard.

"You think he's unstable?" Costello asked.

"I absolutely believe he's unstable," Gallego, an Iraq War veteran, responded.

"Like mentally unstable?" Costello pressed.

"I do believe he's mentally unstable," Gallego replied. "Who in their right mind would attack a Gold Star family? And not only that, once they've actually done that, then turn around and double down, and then accusing a Gold Star mother of not being able to speak because of some weird religious bias that he believes that she had."

Politicians on both side of the political aisle have called on Trump to apologize for his attacks on Khizr and Ghazala Khan, whose son, Capt. Humayun Khan, was killed in Iraq in 2004. Khizr Khan openly challenged Trump in an emotional speech before the Democratic National Convention last week.

Gallego was one of several military veterans in Congress who sent a letter to House Speaker Paul Ryan, R-Wis., urging him to drop his endorsement for Trump over the Khan attacks.

"For politicians like Paul Ryan, Sen. McCain to basically continue to endorse this person, you're only encouraging that behavior. Not only that, you're encouraging other people within society that are supporting Trump to actually support this kind of viewpoint," Gallego said.

In Gallego's own DNC speech last week, he defended McCain, R-Ariz., who has been the victim of a Trump attack, one in which he questioned McCain's POW status.

McCain has condemned Trump's attacks on the Khan family, but has not withdrawn his endorsement of the GOP nominee.