"I have now spent more time in Vietnam than Da Nang Dick Blumenthal, the third rate Senator from Connecticut (how is Connecticut is doing?). His war stories of his heroism in Vietnam were a total fraud — he was never even there," tweeted Trump, who received five draft deferments between 1964 and 1972.
"We talked about it today with Vietnamese leaders!"
Shortly before the second U.S.-North Korea summit in less than a year, Trump met with the Vietnamese prime minister and other officials. He has described the Southeast Asian country as "thriving like few places on Earth."
"We generally decline to waste time engaging with Trump's brazen lies about Senator Blumenthal. Nothing new here," Maria McElwain, a spokesperson for Blumenthal, said when contacted by CNBC on Wednesday.
"But the American people should be concerned about the President of the United States squandering time and energy during sensitive negotiations with a serious nuclear threat peddling falsehoods about a political critic."
Trump's Twitter post resurfaced his frequently used "Da Nang Dick" nickname for Blumenthal, who has faced questions about his military record in the past.
In a speech in 2008, as reported by The New York Times, Blumenthal claimed he had "served in Vietnam." He has since apologized for these remarks, saying he had "misspoken" about his military service during the Vietnam War era "on a few occasions." He served in the Marine Corps Reserves from 1970 to 1976 but didn't go to Vietnam.
More than 3.4 million Americans were deployed to Southeast Asia during the Vietnam War, according to the U.S. Department of Veterans Affairs. More than 57,000 died there.
Trump avoided military service thanks to his deferments — four for education and one for a medical exemption, claiming bone spurs in his heels.
The president drew controversy in 2015 when he criticized the late Sen. John McCain, who spent nearly six years as a POW, enduring torture in a North Vietnamese prison after his plane crashed in enemy territory. "He's not a war hero," Trump said.
"He was a war hero because he was captured. I like people who weren't captured."
In an interview with C-SPAN about the Vietnam War in 2017, McCain took a thinly veiled swipe at Trump over his draft deferments. He sharply criticized Americans from the "highest income level" who were able to obtain draft deferments for ailments like a "bone spur."