Opponents of universal background checks for gun sales sometimes make it sound as if Question 3 on the state ballot exists just to create a pointless inconvenience for law-abiding gun owners or to prevent a friend from loaning out a deer rifle.

But a new study published by the referendum’s supporters shows that the question aims at a much bigger target.

Looking at ads in only two publications — Uncle Henry’s classified magazine and Armslist.com, a national gun sales website that bills itself as “Craigslist for guns” — the researchers found that there were an estimated 3,000 guns a year in Maine available for private sale, none requiring the buyer to pass a background check.

That means someone with a violent criminal record or a history of severe mental illness who would have been stopped before buying a gun at a gun shop or a big-box retailer would have a chance to buy a firearm from a stranger — no background check required. And that risk is repeated all over the state thousands of times a year.

That is the issue that Question 3 opponents will have to address if they are going to be successful at the polls this year. Vague hypothetical examples of individual nuisances don’t stand up against well-established evidence of a very large hole in the public safety system. This study puts the emphasis where it belongs, and it should be the grounds on which this battle is fought.

The vast majority of Americans, including Mainers of all political affiliations, agree that certain categories of people should not be allowed to possess guns. Federal law prohibits gun ownership to people convicted of felonies and domestic violence assault and people who have been involuntarily committed to a mental health facility. Those restrictions have long histories and have been deemed constitutional after judicial scrutiny.

Since 1998, gun buyers have been required to pass an instant background check when purchasing from a licensed gun dealership. This requirement has blocked nearly 3 million sales to prohibited purchasers, including more than 5,500 transactions in Maine. The catch is that many people who sell guns are not required to be licensed, and private sales at gun shows or through classified ads are carried out with no background checks.

How often does a criminal use this loophole to buy a gun? It’s very hard to measure. Portland police know of at least one weapon that was involved in two murders shortly after being sold in a private sale. Because no background check was conducted, it’s impossible to know whether that sale involved a prohibited buyer.

But we do know that if the buyer had been turned down at a gun shop, he could have gone to a gun show the same day to purchase the very same thing that the law says he is prohibited from owning. That’s like a bar that checks IDs at the front door but lets people of all ages in through the back.

The current law provides thousands of opportunities for a felon, a domestic abuser, a fugitive or someone who has been adjudicated as mentally ill to acquire a gun. Some private dealers say they wouldn’t sell to strangers, but of the thousands of guns advertised for private sales, there is ample opportunity to avoid the law.

Opponents of the referendum should be able to explain why it’s OK to enforce a law that most people support only some of the time.

This is not a question regarding the constitutional right to own guns, local hunting traditions or convenience. It’s a question of whether it serves public safety to maintain such a gaping hole in the law that keeps guns out of the wrong hands.

Arguments against Question 3 should attempt to answer that.

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