Washington voters approved Initiative 1639, the most sweeping piece of gun safety legislation put before voters in recent history. I-1639 is now one of the strictest gun laws on the books nationally.

About 60.3 percent of voters approved I-1639 as of Thursday evening.

RELATED: Track election results

I-1639 raises the purchase age to 21 for semi-automatic rifles, creates enhanced background checks for rifles to be in line with the checks for handguns in the state, and creates new standards for secure storage.

Alan Gottlieb, founder of the Second Amendment Foundation, said he’s not surprised with the initial results on I-1639.

“We will challenge it in court,” said Gottlieb.

WATCH: Election Night recap on KING 5

RELATED: Tracking Washington’s hot races in November election

Supporters of the measure say it's a major step forward for addressing factors in mass shootings and would help reduce suicide deaths by firearms and accidental shootings.

Opponents are concerned the law would curb gun owners’ rights. They also say the initiative is flawed, because it is not directly tied to public safety.

RELATED: Debating I-1639 on gun safety

Washington Attorney General Bob Ferguson said the opposition has been clear they would file a lawsuit challenging the initiative.

“For high-profile initiatives, there is almost invariably a lawsuit," Ferguson said. "That’s the way the world works these days, and we’re prepared for that.”

If there is a lawsuit, Ferguson, would endorse I-1639, would be tasked with defending the initiative.

The initiative faced a court battle before it even got to the ballot when in August a judge tossed out 300,000 signatures to get the initiative on the ballot, saying the petitions didn’t clearly mark which portion of the law would change. However, the state Supreme Court overturned that ruling a week later finding it inappropriate to base its decision just on the formatting of the petition.

WATCH: Student movement behind gun safety effort