Blumenthal's campaign on Monday night called a report on the New York Times website, which includes video of him at a 2008 event saying he had served "in Vietnam," an "outrageous distortion" on his record.
"Unlike many of his peers, Dick Blumenthal voluntarily joined the Marine Corps Reserves and served for six months in Parris Island, S.C. and six years in the reserves," Mindy Myers, Blumenthal's campaign manager, said in a written statement. "He received no special treatment from anyone."
Times spokeswoman Diane McNulty told The Associated Press, "We stand by the story."
Blumenthal told The Times that he's always tried to make it clear that his Marine Reserve service never took him overseas. The Times reviewed documents which showed the Democrat - who has been the front-runner in the Senate race - got five deferments to avoid going to war between 1965 and 1970.
Blumenthal told the Times he had misspoken at the 2008 event in Norfolk in which he said he served in Vietnam.
In a televised March debate, Blumenthal stated clearly he had not actually served in Vietnam during the conflict when asked a question about using military force in Iran.
When called at home on Monday, Blumenthal's wife, Cynthia, said her husband was out-of-state. Blumenthal did not immediately returns messages left on his cell phone.
Blumenthal has planned a news conference with Connecticut veterans on Tuesday, but no details have been released.
Questions about Blumenthal's military service could have political reverberations. It comes days before Connecticut Democrats meet at their party convention on Friday night to endorse a candidate to fill the retiring U.S. Sen. Chris Dodd's seat. Blumenthal is facing a challenge for the nomination from Mystic businessman Merrick Alpert, but is expected to easily win the party's endorsement.
His two potential Republican rivals immediately jumped on the news.
"It's very clear to us, over the past few weeks and months as we've begun to research Mr. Blumenthal in earnest, there are some deeply troubling discrepancies between the image he's portrayed publicly and the truth," said Ed Patru, a spokesman for former wrestling executive Linda McMahon, who is seeking the GOP nomination.
Former U.S. Rep. Rob Simmons, a Vietnam veteran who is also seeking the party's endorsement, said he was "deeply troubled by allegations that he has misrepresented his service. Too many have sacrificed too much to have their valor stolen in this way," he added.
The Democratic Senatorial Campaign Committee accused McMahon's campaign of being behind the Times story.