Being on vacation hasn't kept President Trump from going on the Twitter attack.

On Monday, the president criticized Democratic Sen. Richard Blumenthal of Connecticut. The senator had appeared on CNN that morning, discussing the Justice Department's crackdown on leaks, the sanctions on North Korea and what he called "potential collusion" between the Trump campaign and Russia.

"That investigation must be pursued," Blumenthal, a member of the Senate Judiciary Committee, said of the ongoing Russia probes.

That drew the president's ire.

"Interesting to watch Senator Richard Blumenthal of Connecticut talking about hoax Russian collusion when he was a phony Vietnam con artist!" the president said in a series of tweets. "Never in U.S. history has anyone lied or defrauded voters like Senator Richard Blumenthal. 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?"

Several hours later, he continued with his rant: "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."

Blumenthal did indeed exaggerate about his military service, including one notable occasion in 2008, when he told a group of veterans and senior citizens, "We have learned something important since the days that I served in Vietnam." He got multiple military deferments during the Vietnam War before landing in the Marine Reserve. He then served in the reserve from 1970 to 1976.

While attacking Blumenthal over Vietnam isn't new for Trump — he's done it twice before this year — Trump also got multiple deferments during the Vietnam War, and he never served in the military.

Blumenthal shot back at the president's morning tweets.

"Mr. President: Your bullying hasn't worked before and it won't work now," he wrote. "No one is above the law."

Blumenthal co-sponsored legislation last week designed to protect special counsel Robert Mueller, the head of DOJ's Russia probe.