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Donald Trump.
The president rehashed his argument that Blumenthal is a "con artist" for claiming to have served in the Vietnam War, when, in fact, the senator deferred overseas military service and instead served domestically in the reserves.
"I think Senator Blumenthal should take a nice long vacation in Vietnam, where he lied about his service, so he can at least say he was there," Trump wrote just before 5 p.m. ET.
This comes after Trump sent a series of tweets on Monday morning attacking Blumenthal's credibility just minutes after the senator criticized the president and his administration during a CNN interview.
"Interesting to watch Senator Richard Blumenthal of Connecticut talking about hoax Russian collusion when he was a phony Vietnam con artist!" Trump wrote just before 8 a.m. ET, referring to reporting in 2010 that Blumenthal had repeatedly stated or implied he had served in Vietnam when in fact he obtained five military deferments.
Trump's tweets came soon after the CNN interview in which the senator accused the administration of "politicizing" the Department of Justice and argued that the investigation of possible collusion between the Trump campaign and Russia "must be pursued."
"Never in U.S. history has anyone lied or defrauded voters like Senator Richard Blumenthal," Trump continued. "He told stories about his Vietnam battles and conquests, how brave he was, and it was all a lie. He cried like a baby and begged for forgiveness like a child. Now he judges collusion?"
Blumenthal appeared on CNN for a second time on Monday, just minutes after Trump's afternoon tweet, and argued that the attacks are a distraction from the Russia investigation and issues of national security.
"There is an ongoing special counsel investigation - it is real, it is based on fact," Blumenthal told CNN's Wolf Blitzer. "That's what is important. That focus is what really matters."
Earlier on Monday, the senator accused Trump of bullying him.
"Mr. President: Your bullying hasn't worked before and it won't work now. No one is above the law," Blumenthal tweeted. "This issue isn't about me - it's about the Special Counsel's independence and integrity."
This isn't the first time the president has retaliated against the senator for his criticism of the administration.
In February, Trump accused Blumenthal of misrepresenting remarks that Neil Gorsuch, then a nominee for the Supreme Court, made to the senator concerning Trump's attempts to delegitimize a federal judge. Trump retaliated against Blumenthal on Twitter, bringing up his "major lie" about his military record.
Trump attacked Blumenthal again in May after the senator criticized his decision to fire FBI Director James Comey. Repeating his attacks, Trump accused Blumenthal of devising "one of the greatest military frauds in U.S. history" and argued that the senator "should be investigated for his acts."