Sen. John Walsh, a Democrat, dropped out of Montana’s Senate race Thursday after accusations of plagiarizing a research paper at the U.S. Army War College in 2007 proved to be too much for his campaign to handle.

Last month, Walsh admitted to the charges detailed by the New York Times, saying the copying was an “unintentional mistake,” but the scandal began to overshadow his campaign, he said in a statement on Thursday.

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

Walsh, who had been lieutenant governor, was appointed Feb. 7 by Gov. Steve Bullock to replace longtime Sen. Max Baucus, D-Mont., who resigned to become ambassador to China. He will continue to serve as senator until January 2015.

The research paper in question was a thesis on American policy in the Middle East. In it, Walsh lifted entire paragraphs from other papers on the issue, including about 800 words verbatim from a 2002 paper from the Carnegie Endowment for International Peace.

Walsh served in the National Guard for 33 years and led the largest deployment of Montana’s National Guard since World War II (to Iraq).

Last week, Walsh began canceling campaign events, and following what his campaign spokeswoman called a “personal day” on Tuesday, his entire campaign went into lockdown. That ended with the announcement that he was withdrawing from the race.

Walsh’s replacement on the ticket has not been announced, but according to the Montana Democratic Party’s Rule 15, a nomination convention must be held where leaders from all over the state would get a chance to vote. The Montana Democratic Party has until Aug. 20 to name a replacement.

The Republican nominee, Rep. Steve Daines, was already leading Walsh before the scandal broke.