HELENA — U.S. Sen. John Walsh of Montana is dropping his campaign for office amid allegations that he plagiarized large portions of a 2007 research project he wrote for a master’s degree, a Democrat close to the former National Guard commander’s campaign said Thursday.

The source spoke on condition of anonymity because they were not authorized to discuss Walsh’s plans.

In a statement to supporters obtained by Lee Newspapers of Montana, Walsh said he is leaving the race but will keep the seat he was appointed to until his term ends in January 2015, when the winner of November’s election is sworn in.

“I am ending my campaign so that I can focus on fulfilling the responsibility entrusted to me as your U.S. senator,” the Democrat’s statement said. “You deserve someone who will always fight for Montana, and I will.”

The news comes as a U.S. Army War College investigation is set to begin Aug. 15 into the paper that Walsh previously said unintentionally contained wrongfully cited passages.

His decision to withdraw from the November election allows the Montana Democratic Party to hold a nominating convention to choose a replacement candidate before the Aug. 20 deadline to do so. The convention will be comprised of Democratic leaders from each county’s party committee, along with federal and statewide elected officials and the party’s executive board. They will nominate potential candidates who will have a chance to speak before the convention before voting gets underway.

The nominee who receives a majority of votes will be selected as the replacement candidate, according to the party’s rules.

Walsh decision is likely to give a boost to Republican Rep. Steve Daines, who is giving up his House seat to run for Senate. Republicans need to gain a net of six seats in the election to take control of the Senate, and Walsh had faced a tough race against Daines before the plagiarism allegations.

Walsh had already announced his candidacy for the seat when Gov. Steve Bullock appointed him in February to replace Max Baucus, who resigned from the Senate to become ambassador to China. Republicans blasted Bullock’s appointment of his lieutenant governor as a political move designed to gain an advantage in the elections.

The New York Times revealed the extensive use of unattributed material in Walsh’s paper about the spread of democracy in the Middle East. Walsh originally called it an “unintentional mistake” and told The Associated Press part of the blame may lie in his being treated for symptoms of post-traumatic stress disorder following his deployment in Iraq.

He later said he was not blaming PTSD for his mistake.

The pressure on Walsh’s campaign grew after the revelations, with the editorial boards of Montana’s three largest daily newspapers calling for him to withdraw his candidacy over the past two weeks.

“If the Democrats want to hold on to the Max Baucus Senate seat, their best hope is for Walsh to step aside by Aug. 11 and for a new candidate to take his place on the ballot for a fresh start,” the Great Falls Tribune wrote on July 27.

Walsh is the only U.S. senator who served in the Iraq war. He capped a 33 years in the Montana National Guard, his career rising to state adjutant general before he took his first elected office in 2013 as Bullock’s lieutenant governor in 2013.

Walsh received the Master of Strategic Studies degree from the war college at age 47, a year before he became adjutant general overseeing the Guard and the state Department of Military and Veterans Affairs.

Associated Press writer Philip Elliott in Washington contributed to this report.