Blumenthal on CNN discussed the Russia investigation and the prospect of "politicizing" the Justice Department. He pointed to the potential chilling effect of the administration's pledge to crack down on leaks.
Trump, who is on what he calls a working vacation at his golf club in New Jersey, apparently watched the segment and hit back at Blumenthal on Twitter. The president targeted Blumenthal for saying he served in Vietnam when he did not.
Trump wrote: "Interesting to watch Senator Richard Blumenthal of Connecticut talking about hoax Russian collusion when he was a phony Vietnam con artist!"
Trump referred to the investigation into Russian attempts to influence the 2016 election and whether his campaign colluded with the Kremlin. The president has repeatedly called the probe a "witch hunt."
Trump went on to say that Blumenthal "defrauded voters" and "cried like a baby and begged for forgiveness like a child."
In a subsequent tweet, Blumenthal said Trump's "bullying hasn't worked before and it won't work now."
"No one is above the law," he added.
When Blumenthal was running for Senate in 2010, The New York Times reported that he got "at least five military deferments from 1965 to 1970 and took repeated steps that enabled him to avoid going to war." In 2008, he had spoken of serving in the Vietnam War. Blumenthal did serve in the Marine Corps Reserve.
Trump also claimed that Blumenthal "told stories about his Vietnam battles and conquests."
The president himself received five military deferments exempting him from serving in Vietnam.
Shortly before he tweeted about Blumenthal's service record, Trump called the Times "totally inept."
Trump continued his social-media attack on Blumenthal later Monday, suggesting that the senator take a vacation in Vietnam.