The senator, then a U.S. Army commander, took nearly a quarter of his paper from other sources without citing them, the Times finds, including six recommendations at the end of the paper. The 800-word section is copied nearly verbatim from a paper from the Carnegie Endowment for International Peace.
Additional sections of the paper seem to be copied from a paper from a Harvard research institute.
Walsh told the Times that he "didn't believe" that he had plagiarized.
"I didn't do anything intentional here," he said.
Walsh's campaign says that he unintentionally didn't properly cite the sources. The campaign said he was also going through a rough patch while he wrote the paper. They said he'd just returned from deployment in Iraq and was dealing with the suicide of a soldier he'd commanded shortly before the assignment was due.
"This was unintentional and it was a mistake," says Walsh spokeswoman Lauren Passalacqua. "There were areas that should have been cited differently but it was completely unintentional.
Senator Walsh released every single evaluation that he received during his 33-year military career, which shows an honorable and stellar record of service to protecting Montana and serving this country in Iraq," she added.
The case could hurt Walsh, who recent polls indicate has been cutting into Daines's lead in the race.