The National Rifle Association filed a lawsuit accusing its longtime advertising agency Ackerman McQueen Inc. of refusing to comply with demands to justify its billings, an extraordinary public break with the gun-rights group’s largest outside partner.

The lawsuit, filed late Friday, comes amid an unusual battle unfolding behind the scenes at the NRA’s 76-member board, which some say pits a small group of pro-Ackerman McQueen directors against other board members and an outside NRA attorney.

The dispute in part is about how the NRA, with an annual budget of more than $300 million, is spending money during a period when its finances have been tight. The NRA ran at a deficit in its two most recently reported years.

Oklahoma City-based Ackerman McQueen has been the NRA’s ad agency since the 1980s and has been widely credited with helping to transform the NRA from a grass-roots operation to a powerful national advocacy group. In recent years the ad firm has also produced the organization’s NRATV, a video outlet that mainly focuses on conservative and pro-gun rights commentary.

NRA filings show it paid Ackerman McQueen $42.6 million in 2017, the most recent year available, making it by far the group’s largest vendor.