(CNN) - Sen. John Walsh dropped out of the Senate race in Montana on Thursday amid allegations of plagiarism, a senior Democratic source familiar with the senator's thinking confirmed to CNN.
The move further hurts Democratic chances of maintaining control of the Senate. Republicans need a net gain of six seats in November to take the majority.
The Democrat is accused of not properly attributing certain material in a paper to complete his master's degree at the Army War College.
The New York Times first reported last month that a review of a 2007 final paper suggests the decorated Iraq war veteran "appropriated at least a quarter of his thesis on American Middle East policy from other authors' works, with no attribution."
Material from a Carnegie Endowment for International Peace report and a 1998 essay by a Harvard scholar were included in the paper without proper sourcing, the Times reported.
The Pentagon announce last week its inspector general will review the college's academic investigation of the plagiarism issue.
Walsh's reelection campaign acknowledged "the citations were not all done correctly," but insisted it was an "unintentional mistake."
He later told the Associated Press he was suffering from post-traumatic stress after his Iraq deployment.
Walsh's decision to pull out of the race was first reported by the Billings Gazette.
In a letter to supporters obtained by the newspaper, Walsh said: “I am ending my campaign so that I can focus on fulfilling the responsibility entrusted to me as your U.S. senator. … You deserve someone who will always fight for Montana, and I will.”
Walsh was already facing a tough reelection challenge in the conservative state against GOP candidate Rep. Steve Daines.
It's "highly unlikely" that former Montana Gov. Brian Schweitzer, who declined to run for the seat earlier this year, will jump in, according to the same senior Democratic source.
Possible Democratic candidates who might run, according to a Montana Democratic source, are Nancy Keenan, the former president of the reproductive rights organization, NARAL Pro-Choice America; state Sen. Dave Wanzenried or state Rep. Franke Wilmer.
Montana law states that candidates must drop out at least 85 days before Election Day in order to be replaced on the ballot. It's up to the state party's central committee to select a replacement.
Walsh stepped down as Montana's lieutenant governor after he was appointed to the Senate in February to replace Max Baucus, who is now ambassador to China.
CNN's Dana Davidsen contributed to this report