OLYMPIA — A federal judge Monday upheld a sweeping set of firearms regulations approved by Washington voters in 2018, ruling against a legal challenge brought by gun-right advocates.

Initiative 1639, among other things, raised the legal purchase age of a semiautomatic rifle to 21 and put in place enhanced background checks for their purchase. It also barred the sale of such rifles to out-of-state residents.

While voters passed the measure with nearly 60% support, gun-rights advocates have fiercely opposed it. At the same time, some sheriffs in Washington counties have said they wouldn’t enforce the law because they believed it was unconstitutional.

But U.S. District Court of Western Washington Judge Ronald Leighton cited current federal law banning the sale of handguns to people under 21, as well as state laws going back to the 19th century that have imposed age restrictions on purchases.

Because, for much of the nation’s history, people between the ages of 18 and 20 were considered minors, several courts have ruled that age restrictions fall outside the Second Amendment’s protections, he wrote.

“These authorities demonstrate that reasonable age restrictions on the sale, possession, or use of firearms have an established history in this country,” Leighton wrote in the order. “The extension of Washington’s age restrictions” for semiautomatic rifles “is ultimately a distinction without a difference.”

The lawsuit by several plaintiffs, including two Washington firearms dealers, the National Rifle Association (NRA) and Bellevue-based Second Amendment Foundation, argued that I-1639 violates the Second Amendment, for barring 18-to-20 year olds from buying semiautomatic rifles.

They filed the lawsuit against Spokane Police Chief Craig Meidl, Clark Clark County Sheriff Chuck Atkins and Teresa Berntsen, director of the state Department of Licensing.

Those are three public officials who could revoke the plaintiff gun dealers’ federal firearms licenses were they to sell semi-automatic assault rifles to out-of-state residents, according to court documents.

Alan Gottlieb, founder of the Bellevue-based Second Amendment Foundation, said the ruling was not a surprise and the group plans to appeal to the U.S. Court of Appeals for the Ninth Circuit.

“The judge doesn’t think that 18 to 20-year-olds have rights,” Gottlieb said. “A fundamental constitutional right should definitely apply to them as well.”

In a statement, Keely Hopkins, NRA’s Washington state director, blasted the decision.

“It’s disappointing but not surprising to learn that a sitting judge chose to blatantly disregard the constitutional infringements ballot initiative 1639 imposes upon law-abiding gun owners,” Hopkins said in prepared remarks. “Now more than ever it is imperative that those who choose to exercise their Second Amendment rights are not denied this fundamental freedom no matter where they call home.”

Long guns such as rifles or shotguns using manual operations to chamber rounds — including pumps, slides, bolts or levers — are still available for legal purchase at age 18.

Advocates for stricter gun laws hailed Monday’s ruling in a legal challenge that has simmered since February 2019.

In a statement, the head of the Alliance for Gun Responsibility, the group that campaigned for I-1639, applauded Leighton for upholding “commonsense provisions.”

“We are glad that Judge Leighton upheld these commonsense provisions that are designed to keep our schools and communities safe from gun violence,” said Renée Hopkins, chief executive officer for the Seattle-based group. “The gun lobby lost in the court of public opinion two years ago, and this lawsuit was a last-ditch effort to block these lifesaving policies.”

Similar to handgun purchases, I-1639 requires local law-enforcement jurisdictions to conduct enhanced background checks to make sure someone is legally eligible to possess a firearm.

After some sheriffs said they would not enforce the law, Attorney General Bob Ferguson wrote an open letter, saying they could be held liable for refusing to conduct the checks if someone not allowed to buy a gun was able to purchase one and use it for a crime.

In a statement Monday, Ferguson said he was “confident that Washington law enforcement officials will carefully review this ruling from a Bush-appointed federal judge upholding the constitutionality of I-1639.”

“It should not take another letter from me to convince them to do their jobs,” wrote Ferguson in prepared remarks, adding later: “Enhanced background checks will save lives, and the duty to perform these checks is not discretionary.”

In his order, Leighton also upheld a provision of I-1639 that bars the sale of semi-automatic rifles to out-of-state residents.

Gun-rights advocates, in the challenge, argued that component violated federal prohibitions saying state laws can’t discriminate against international or interstate commerce.

But those prohibitions — which come from what is known as the U.S. Constitution’s commerce clause — are geared toward instances where states are engaging in economic protectionism, Leighton wrote.

The law’s ban on out-of-state sales doesn’t trigger that protectionism concern because it neither benefits Washington’s economic interests nor burdens other state’s economic interests, Leighton wrote.

Staff reporter David Gutman contributed to this piece.