"When people come back from war and combat, they see things that maybe a lot of the folks in this room have seen many times over, and you're strong and you can handle it," Trump said to a veterans' group in Northern Virginia, "but a lot of people can't handle it."
Biden laid into Trump on Monday night, referring to his late son Beau's service in Iraq.
"Where in the hell is he from?" Biden said, before pointing to Beau and his own 29 visits to Iraq and Afghanistan. Biden then relayed a story.
"I found myself in Iraq being asked by General [Raymond T.] Odierno — a four-star — to pin a silver medal on a young captain who had pulled someone out of a burning Humvee, risking his life," Biden said. "And when I went to pin it on him in front of the entire brigade, he said to me — looked at me and said, 'Sir, I don't want the medal. I don't want the medal.' You know why? He said, 'He died. He died, Mr. Vice President. I don't want the medal.'
"How many nights does that kid go to sleep seeing that image?" Biden said, beginning to yell. "Dealing with it?"
In an interview airing Tuesday morning on CNN, Biden continued the attack.
"That kid probably goes to sleep every night with a nightmare," he said. "And this guy doesn't understand any of that? How can he not understand that? How can he be so out of touch? He's not a bad guy. But how can he be so out of touch and ask to lead this country?"
And: "This is an ignorant man. This guy says things he has no idea about. He's not a bad man. But his ignorance is so profound. So profound."
Trump's comments Monday drew criticism, given how veterans groups are trying to reduce the stigma associated with mental health issues.
Biden hasn't weighed in on the presidential race often, but when he has, he has been about as effective as it comes in painting Trump as a charlatan who would be completely out of his element as president.
Trump's gaffe opened the door wide to just such a criticism on Monday, and Biden was about the best surrogate Clinton could have had at her rally that night.