Sen. John Walsh plagiarized his final thesis for his master's degree at the Army War College from sources at the Carnegie Endowment for International Peace, and a 1998 essay written by Sean M. Lynn-Jones, a scholar at Harvard's Belfer Center for Science and International Affairs, according to an analysis by the New York Times.

The analysis found nearly a third of the paper was composed of material either similar or verbatim to other sources, and is presented without attribution. Another third uses the exact language of other sources, but is attributed through footnotes. However, the language is often presented without quotation marks, a practice forbidden by War College rules.

“Plagiarism is a serious form of cheating that carries serious consequences,” according to the War College handbook. “Sooner or later academic dishonesty will be discovered.

“Copying a segment of another’s work word for word, then conveniently ‘forgetting’ to include quotation marks but ‘remembering’ to cite the source,” is regarded by the handbook as a form of academic fraud. It may also be prosecutable under military law.

On Tuesday, when presented with the evidence, Walsh told the Times he didn’t believe he did anything wrong. “I didn’t do anything intentional here,” he said. He also claimed he did not recall consulting Carnegie or Jones’s work. As to whether he plagiarized, Walsh said “I don’t believe I did, no.”

On Wednesday a Walsh campaign aide told the Times that the incident should be taken in the context of his extensive service to the country. She also noted that Walsh “dealt with the experience of post-deployment” during his time at the War College. The senator served in Iraq from 2004-2005. Additionally, a member of his unit committed suicide weeks before the paper was due.

The incident is not the first time Walsh has misrepresented his academic record. In a biographical directory of members of Congress, Walsh claimed to have matriculated at the University at Albany, State University of New York in 1990. In fact, he earned a bachelor of science from Regents College, an adult-learning institute that issued degrees under the auspices of the SUNY system. Walsh changed the listing after a Roll Call article called attention to the discrepancy.

The development is a troubling one for Senate Democrats. Walsh is widely considered the most vulnerable incumbent Democrat in the 2014 election cycle, and currently trails his opponent, Republican Rep. Steve Daines. The University of Virginia's Center for Politics says the race “Leans Republican” though recent polls found the race had tightened considerably.