Sen. John Walsh (D-Mont.) blatantly copied from or included without proper attribution the work of other writers in his thesis paper for a master's degree from the United States Army War College, according to an investigative report by The New York Times' Jonathan Martin. The case appears to be pretty damning; the Times has a color-coded version of the thesis detailing all of the instances of alleged plagiarism.
Walsh wrote the thesis in 2007, so this would not be a case of a foolish teen undergrad screwing up.
That said, the revelation should have little impact on the midterms because Walsh already had almost zero hope of holding the seat to which he was appointed earlier this year. The latest Times forecast gave him a six percent chance of winning in November.