Sen. Lisa Murkowski’s campaign issued a sharp rebuttal Thursday to ads from Sen. Mark Begich touting their close cooperation, sending the Begich campaign a cease-and-desist letter demanding an end to them.

Sen. Mark Begich told reporters Thursday afternoon that his campaign plans to keep running the "Great Team" TV commercials, saying their content was factual.

The letter from Holmes Weddell and Barcott, the law firm representing the Murkowski campaign, is addressed to Begich’s campaign manager Suzanne Fleek-Green and Max Croes, his communications director. It blasts the Begich campaign’s ad as violating federal law and containing uses of Murkowski’s image without asking her permission in advance, as well as false and misleading statements.

“Your advertisement prominently features a photograph of the senators in Senator Murkowski’s official Senate office,” lawyers wrote. “As you are aware, use of official Senate resources to assist Senator Begich’s campaign is strictly prohibited by U.S. Code and Senate ethics rules. Senator Murkowski condemns such improper use of official resources.”

The letter also says the voice heard in the ad makes a number of claims that are untrue on closer examination.

“The narrator of the advertisement, Skip Nelson, claims to be a ‘lifelong Republican’ who voted for Senator Murkowski in 2010, and is voting for Begich this fall,” lawyers wrote. “In reality, Mr. Nelson has never been registered as a Republican in Alaska -- he has been registered ‘undeclared’ since 2000. Additionally, Mr. Nelson did not vote at all in 2010.”

Murkowski’s campaign calls for the Begich campaign to stop running and never again air the ad, remove it from any websites attached to its campaign website, and refrain from “any other message that uses Senator Murkowski’s image or name, while implying her support.”

“Thank you in advance for your immediate compliance with this letter,” lawyers wrote. “Please be advised that any future false advertisements naming Senator Murkowski, or any further misuse of her image, will not be tolerated.”

The Begich ads have aired as Alaska’s junior senator awaits the outcome of Aug. 19’s GOP primary to learn which Republican candidate will challenge him for his office. Lt. Gov. Mead Treadwell, former state attorney general Dan Sullivan and Joe Miller, who defeated Murkowski for her seat's Republican nomination in 2010 but lost to her write-in bid in the general election, are all vying to take Begich on.

Murkowski’s director of communications, Matthew Felling, declined to comment on the letter, saying he found out about it Thursday at the same time it was released to the media. Asked about how the matter might affect the working relationship between Murkowski and Begich, he described their work to date as a matter of pragmatism.

“The Senate is a small body of 100 individuals, and you work with those around you,” Felling said. “I think the senator believes that if she had a Republican counterpart, Alaska’s interests would be better served.”

Begich’s press secretary, Heather Handyside, similarly declined comment on the letter Thursday. She characterizes the senators’ working relationship as generally productive, noting their close cooperation on issues such as high-level Arctic policy appointments and a recent push by the state’s congressional delegation for the Air Force to base F-35 Joint Strike Fighters at Eielson Air Force Base in Fairbanks. In a success for the campaign, the Air Force named Eielson as its preferred site to base F-35s Thursday.

“Senator Begich and Senator Murkowski have a very cordial relationship,” Handyside said. “The delegation has all worked together to bring the F-35s here.”

Begich is scheduled to speak about the F-35 initiative with a Rotary Club delegation in Anchorage at 1 p.m. Thursday. Handyside says Begich will discuss both the F-35s and the Murkowski campaign’s cease-and-desist letter with reporters shortly afterward.

This is a developing story. Please check and the Channel 2 newscasts for updates.

Channel 2’s Mallory Peebles contributed information to this story.