HELENA, Mont. (AP) — Montana Sen. John Walsh's thesis written to earn a master's degree from the U.S. Army War College contains unattributed passages taken word-for-word from previously published papers.

The Democrat said Wednesday he was on medication and being treated for post-traumatic stress disorder from his service in Iraq when he wrote the paper. He said he also was dealing with the stress of a fellow veteran's recent suicide.

Walsh said he made an unintentional mistake and did not intend to plagiarize.

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

The senator is running against Republican Rep. Steve Daines to keep the seat Walsh was appointed to in February when Max Baucus resigned to become U.S. ambassador to China, and national Democrats said Wednesday they remained "100 percent behind Sen. Walsh."

The apparent plagiarism in Walsh's 2007 thesis, titled "The Case for Democracy as a Long Term National Strategy," was first reported by The New York Times in a story posted online Wednesday afternoon. Walsh submitted the paper to earn his Master of Strategic Studies degree 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, reviewed by the AP after the Times posted its story and graphics online, 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 lists 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 of the paper 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.

Lynn-Jones said Walsh appears to have received a degree on the basis of work that was not entirely his, 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 for comment to the U.S. Army War College rang unanswered Wednesday.

Walsh said he would consider apologizing to the scholars. He declined to answer repeated questions on 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.

Walsh 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 family and his doctors, 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. "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.

Daines spokeswoman Alee Lockman said she had just seen the Times' report and did not have an immediate comment.