President Trump attacked Sen. Richard Blumenthal, D-Conn., on Twitter Monday night, mockingly referring to him as "Da Nang Dick" and questioning his fitness to serve on the Senate Judiciary Committee in the wake of decade-old allegations of stolen valor related to Blumenthal's false claim that he fought in the Vietnam War.

"How does Da Nang Dick (Blumenthal) serve on the Senate Judiciary Committee when he defrauded the American people about his so called War Hero status in Vietnam, only to later admit, with tears pouring down his face, that he was never in Vietnam," wrote Trump, who added that Blumenthal was, "An embarrassment to our Country!"

It's unclear exactly what prompted the president's tweet. Earlier Monday, Blumenthal and Sen. Chuck Grassley, R-Iowa, introduced legislation that would require Special Counsel Robert Mueller to submit a report to Congress and the public when his investigation into alleged collusion between Russian officials and the Trump campaign concludes. The legislation also would require a report within two weeks if a special counsel is fired, transferred or resigns.

Blumenthal, who was elected to the Senate in 2010, regularly referenced his supposed Vietnam service in the 2000s, when he was Connecticut attorney general.

“I served during the Vietnam era,” Blumenthal reportedly said at a Vietnam War memorial in 2008. “I remember the taunts, the insults, sometimes even the physical abuse.”

Blumenthal reportedly obtained at least five military deferments between 1965 and 1970. He eventually served in the U.S. Marine Corps Reserve, but did not deploy to Vietnam.

In 2010, Blumenthal admitted that he had "misspoken about my service, and I regret that and I take full responsibility."

Grassley and Blumenthal are both members of the Senate Judiciary Committee and Grassley is a former chairman of the panel. Both men supported legislation last year to protect Mueller's job. The bill, approved by the Judiciary Committee in April, would allow any fired special counsel to seek a judicial review within 10 days of removal and put into law existing Justice Department regulations that a special counsel can be fired only for good cause.

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"A special counsel is appointed only in very rare serious circumstances involving grave violations of public trust," Blumenthal said. "The public has a right and need to know the facts of such betrayals of public trust."

Senate Majority Leader Mitch McConnell, R-Ky., declined to hold a vote on the bill, however, saying it was unnecessary.

Fox News' Brooke Singman and The Associated Press contributed to this report.