Washington (CNN) It was a moment from the early days of the US invasion of Iraq that 2020 Democratic presidential candidate Seth Moulton says still haunts him.

Moulton, who served four tours with the Marines in Iraq, opened up about his own experience with post-traumatic stress, revealing how he still thinks about a painful decision to leave a wounded Iraqi boy behind. He also highlighted his policy proposals to help millions of veterans and Americans with mental health in an interview with Jake Tapper on CNN's "State of the Union" that aired Sunday.

"The Marines just a few hundred yards ahead of us shot up some cars and buses that they thought were full of enemy troops. But at least one car was an Iraqi family, just fleeing the violence," the Massachusetts Democrat told CNN. "There is a boy, probably about five years old, lying in the middle of the road, wounded and writhing in pain."

"At that moment, I made one of the most difficult decisions of my entire life, which was to drive around that boy and keep pressing the attack because the stop would have stopped the entire battalion's advance. It would have endangered the lives of dozens, if not hundreds, of Marines," he said. "But there is nothing I wanted to do more at that moment than just get out of my armored vehicle and help that little kid."

Moulton said that scene has stuck with him long beyond the war, telling Tapper, "There was a time when I got back from the war when I couldn't get through a day without thinking about that 5-year-old boy, leaving him in the middle of the road."

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