President Donald Trump at the White House. AP Photo/Evan Vucci

President Donald Trump sent out a series of tweets on Monday morning attacking Sen. Richard Blumenthal's credibility just minutes after the Connecticut Democrat criticized the president and his administration on CNN.

"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 and ultimately served in the reserves, never going overseas.

Trump's tweets came soon after a cable news 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 responded later Monday morning, accusing Trump of "bullying" him to distract from questions about the ongoing Russia investigation.

"Mr. President: Your bullying hasn't worked before and it won't work now. No one is above the law," he tweeted. "This issue isn't about me — it's about the Special Counsel's independence and integrity."

This is not the first time the president has attacked the senator. 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."