SEATTLE — Washington state Attorney General Bob Ferguson fired back at sheriffs and law enforcement who have said they would not enforce Initiative 1639, which is one of the nation's toughest gun laws.

In an open letter penned Tuesday, Ferguson wrote that local law enforcement was entitled to their opinions on the law, but that does not give them license to not enforce it.

“If you personally disagree with Initiative 1639, seek to change it,” Ferguson wrote. “Or file a lawsuit challenging it. But do not substitute your personal views over that of the people.”

I-1639 raises the purchase age of semi-automatic rifles to 21, incentivizes safe storage, and creates enhanced background checks for rifles that are line with the checks for handguns in the state. The initiative also authorizes the state to require gun sellers to add $25 to sales of semi-automatic rifles to pay for new regulations.

In his letter, Ferguson cited the new enhanced background checks as an example of police’s obligation to enforce the law for the sake of public safety. He argued that if a dangerous individual were able to access a semi-automatic rifle because police didn’t go through proper protocol, law enforcement could be held liable if that person used the gun to do harm.

“In short, the taxpayers of your city or county assume the financial risk of your decision to impose your personal views over the law,” Ferguson wrote.

Voters across Washington passed the measure by 59.4 percent in November, although county breakdowns show it passed primarily in northwestern Washington. Mason, Grays Harbor, Lewis, Chelan, and Kittitas counties were among those that rejected I-1639.

RELATED: Petition to abolish I-1639 fails to get White House response

Sheriffs in 12 mostly rural Washington counties along with the police chief of Republic have come out and said they would not enforce the measure until its legality was decided by the courts. Another sheriff – Lewis County Sheriff Robert Snaza – said back in December he would not actively seek out violators.

The National Rifle Association and the Second Amendment Foundation have filed a lawsuit challenging the initiative as unconstitutional and infringes on second amendment rights.

However, Ferguson said he was confident the initiative would stand up against any legal challenge, and until the courts rule otherwise I-1639 is presumed constitutional.

“As public officers, our duty is to abide by the will of the people we serve, and implement and enforce the laws they adopt,” Ferguson wrote. “I encourage you to do so.”