Idaho is among 21 states asking a federal appeals court to overturn provisions of Maryland's gun-control law that bans 45 assault weapons and limits gun magazines to 10 rounds.
West Virginia Attorney General Patrick Morrisey led the coalition in filing a friend-of-the-court brief in the Fourth U.S. Court of Appeals in Richmond, Virginia last week.
The brief says the law violates the Second Amendment right to keep firearms in homes for self-protection.
The other states are Alabama, Alaska, Arizona, Florida, Idaho, Kansas, Louisiana, Michigan, Missouri, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Carolina, South Dakota, Texas, Utah, and Wyoming, and Kentucky.
A U.S. District Court judge in Baltimore upheld the provisions in August.
The state of Maryland has until Dec. 31 to respond to the filing.