updated at 3:25pm ET
(CNN) - A significant portion of a paper Montana Sen. John Walsh had to complete to obtain his Master’s degree was taken directly from two sources and given no attribution, according to the New York Times.
At least one-quarter of the 14-page paper, “The Case for Democracy as a Long Term National Strategy,” from the United States Army War College was copied from a Carnegie Endowment for International Peace report and a 1998 Harvard scholar's essay, the Times reported.
Walsh, an Iraq War veteran and former head of Montana’s National Guard, completed the paper in 2007. He was 46.
Nearly 24 hours after the story broke, his re-election campaign released a full-bore defense, laying out more than a dozen accomplishments and awards from his 33-year military career. The campaign didn’t deny the plagiarism but said it wasn’t deliberate.
"He acknowledges the citations were not all done correctly, but that it was an unintentional mistake," the statement said.
Walsh separately told the Associated Press that he was suffering from post-traumatic stress after his Iraq deployment. His office said he commanded an infantry battalion in 2004-05.
His office said the difficult period was not an excuse for the plagiarized portions of the paper.
The damning report could impact not only Walsh’s election chances this November, but also the Democrats’ odds of maintaining control of the Senate.
Walsh stepped down as Montana's lieutenant governor after he was appointed to the Senate in February to replace longtime Democratic Sen. Max Baucus, who was appointed Ambassador to China. Walsh is running in November's midterm election for a full six year term in Senate.
In the conservative state of Montana, Republicans pegged the race as winnable. The GOP must win six seats currently in Democratic hands to win back control of the Upper Chamber.
Even before the plagiarism allegations broke, two of the top non-partisan political handicappers gave the edge in the race to the GOP candidate, Rep. Steve Daines. The Cook Report listed the race as “lean Republican” and the Rothenberg Report rated the contest as “toss up/tilt Republican.”
In a statement, the Democratic Senatorial Campaign Committee defended Walsh.
"John Walsh is a decorated war hero and it’s disgusting that Steve Daines and Washington Republicans are going to try denigrate John’s distinguished service after multiple polls show him gaining. Steve Daines should immediately denounce these latest smears and call for an end to all attacks on John Walsh’s record protecting Montana and serving his country," said DSCC press secretary Justin Barasky.
CNN's Paul Steinhauser contributed to this report.