HELENA, Mont. (AP) — Sen. John Walsh of Montana said Wednesday his failure to attribute conclusions and verbatim passages lifted from other scholars' work in his thesis to earn a master's degree from the U.S. Army War College was an unintentional mistake caused in part by post-traumatic stress disorder.

The apparent plagiarism first reported by The New York Times was the second potentially damaging issue raised this year involving the Democrat's 33-year military career, which has been a cornerstone of his campaign to keep the seat he was appointed to in February when Max Baucus resigned to become U.S. ambassador to China.

National Democrats said Wednesday they remained "100 percent behind Sen. Walsh" in his campaign against Republican Rep. Steve Daines.

Walsh told The Associated Press when he wrote the thesis, he had PTSD from his service in Iraq, was on medication and was dealing with the stress of a fellow veteran's recent suicide.

"I don't want to blame my mistake on PTSD, but I do want to say it may have been a factor," the senator said. "My head was not in a place very conducive to a classroom and an academic environment."

Walsh submitted his thesis, titled "The Case for Democracy as a Long Term National Strategy," to earn his Master of Strategic Studies degree in 2007, nearly two years after he returned from Iraq and about a year before he became Montana's adjutant general overseeing the state's National Guard and Department of Military Affairs.

The paper includes a series of unattributed passages taken from the writings of other scholars.

The first page borrows heavily from a 2003 Foreign Affairs piece written by Thomas Carothers, vice president for studies at the Carnegie Endowment for International Peace, and a 2009 book by Natan Sharansky with Ron Dermer called "The Case for Democracy: The Power of Freedom to Overcome Tyranny and Terror."

Sharansky is a former Soviet dissident and chairman of the Jewish Agency for Israel. Dermer is the Israeli ambassador to the United States.

All six of the recommendations that Walsh listed at the end of his paper are taken nearly word-for-word without attribution from a Carnegie paper written by Carothers and three other scholars at the institute.

One section is nearly identical to about 600 words from a 1998 paper by Sean Lynn-Jones, a scholar at the Belfer Center for Science and International Affairs, a research institute at Harvard. Carothers and a Dermer spokesman declined to comment.

Lynn-Jones said Walsh appears to have received a degree on the basis of work that was not entirely his own, and that anyone seeking credit for an academic degree "needs to acknowledge where the material is coming from."

"Maybe he unintentionally didn't cite my work, but it's up to the Army War College to determine whether this is acceptable by their standards or not," Lynn-Jones said.

An after-hours call to the Carlisle, Pennsylvania, school rang unanswered Wednesday.

Walsh declined to answer repeated questions about whether he believed he earned the degree if the thesis' conclusions were not his own.

"I know about war strategy and defense because of firsthand experience leading a battalion and the Montana National Guard," he said.

The senator said when he wrote the paper, he was seeing two doctors and taking medication to deal with nightmares, anxiety and sleeplessness. He said he has since worked through those issues with his doctors and family, though he still takes antidepressant medication.

Justin Barasky, spokesman for the Democratic Senatorial Campaign Committee, said the committee stands behind 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," Barasky said.

Daines spokeswoman Alee Lockman said she had just seen the Times' report and had no immediate comment.

Even before the plagiarism revelations, top Democratic strategists saw Walsh's campaign as an uphill pull, never counting on it as key to holding their Senate majority.

Republicans need to gain six net seats this fall to control the Senate. South Dakota, West Virginia and Montana are seen as likely GOP pickups, and Republicans have several opportunities to grab the other three contests they need. Top on their lists are incumbent Democrats running in states President Barack Obama lost in 2012: Arkansas, Louisiana, North Carolina and Alaska.

Walsh is the only senator who served in the Iraq war. He capped his long career in the Montana National Guard as the state's adjutant general before becoming lieutenant governor to Gov. Steve Bullock, who appointed him to the Senate seat.

Walsh's military record was first questioned in January when records revealed the Army reprimanded him in 2010 for pressuring guardsmen to join a private association for which he was seeking a leadership role.

Walsh was adjutant general at the time and wanted to become vice chairman of the National Guard Association of the United States. In the reprimand, Army Vice Chief of Staff Peter Chiarelli said he questioned Walsh's ability to lead.

Political scientist David Parker of Montana State University said Walsh's thesis combined with the reprimand raise questions about the senator's integrity.

"If this were it, in isolation, I don't think it would be a big deal," Parker said. "But now we've got two issues of honor and competency."