"I think that Trump believes that by saluting the troops and saluting the flag today it makes up for his unwillingness to do so when it actually matters, his unwillingness to answer the call to go to Vietnam," Moulton said on ABC's "This Week."

"Real service to the country is making a sacrifice to do the right thing for others, to do the right thing for America," he added. "And Trump doesn't understand that at all."

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Trump used his July 4 event to call for young people to join the military, which Moulton, who has made the needs of veterans a central part of his campaign, said was "quite something, coming from someone who refused to do it himself."

Still, Moulton said Trump could be harder to beat in 2020 than many Democrats realize.

"That's why I'm running a campaign based on service. On reclaiming patriotism from right-wing politicians who think they own it," he said. "On what it means from a Democratic perspective to keep America strong and safe."