Imagine that there is a bar in your town with two entrances.

At the front door, a bouncer is checking IDs to make sure that no one under the age of 21 gets in. At the back door, customers can come and go as they please, regardless of their ages.

What might such a policy do to keep alcohol away from kids?

That, in essence, how we enforce the laws around gun sales. Potential customers have a choice: They can buy from a federally licensed firearms dealer, who is required to conduct an instant background check that looks for a history of violent crime or severe mental illness. Or they can opt for a private sale, where no background check is required.

What does that policy do to keep guns out of the hands of people who shouldn’t have them?

Question 3, the measure that would require background checks for all gun sales in Maine, seeks to close that giant loophole and treat all gun buyers the same. We think this is a simple, common-sense solution to a serious problem and that’s why we enthusiastically endorse a “yes” vote.

Well-established federal law lists categories of people prohibited from having a firearm. They include felons, fugitives, illegal immigrants, domestic abusers or people who are under a restraining order for family violence. Also on the list are people who have been involuntarily committed to a mental institution by a judge, or who are illegal users of controlled substances.

Since 1998, gun dealers have been required to run instant background checks for potential buyers, and more than a million of them have failed, including more than 5,000 in Maine. It is impossible to know how many of the people who failed those tests went out for a no-check private sale and got a gun anyway. We also can’t know how many people skipped the test and went straight to a gun show or the classifieds because they knew that they couldn’t pass. But it would be foolish to assume that prohibited persons have not been exploiting this opportunity.

We do know that guns are changing hands in private sales. A study by Question 3 supporters identified 3,000 guns listed for sale in Maine through Uncle Henry’s classified magazine and the Armslist.com website. That number does not include duplicate ads or sellers identified as gun dealers.

Most of the buyers and sellers are law-abiding people. But guns found at crime scenes here and in other states have been traced back to private sales in Maine in which there were no background checks. Notably, the gun used to kill Darien Richardson, who was shot during a home invasion in Portland in 2010, was recovered after it had been used in another homicide. Unfortunately, it had been sold in a private sale with no background check, so the trail went cold and Richardson’s murder remains unsolved.

Opposition to the referendum focuses on two major areas: that the law wouldn’t do any good and that it would disrupt practices that are traditional in Maine. We were not convinced that either is reason enough to justify the current system.

Critics claim that since it is already illegal for a criminal to buy a gun or to have someone else buy a gun for them, only law-abiding Mainers would be affected if background checks were extended to private transfers. The law, they argue, would make it more difficult and expensive for honest people to do what they have every right to do, but it wouldn’t stop criminals from committing crimes.

But that’s exactly why background checks shouldn’t be a matter of choice. It’s like checking IDs at the entrance to a bar: A mandatory background check prevents some people from breaking the law even if that’s what they want to do. Background checks are a proven way of keeping guns out of the hands of people who are not supposed to have them.

Another common criticism of the referendum is that the law would criminalize acts that are currently part of Maine’s sporting culture, especially lending guns to friends.

If Question 3 prevails, friends shooting together could still lend each other guns, and family members could continue to share and pass on guns without a background check. The law would only prevent an unrelated person from borrowing a gun to use outside its owner’s presence.

That may force some people to change their habits, but it’s a small price to pay if it would spare people like the Richardson family the heartbreak of burying a child.

Question 3 would not create any new regulations – it would simply enforce existing laws more effectively. As long as background checks are required in some gun sales, it makes no sense to exempt them in so many others.

We should not leave a back door open for criminals and people with severe mental illness who want to buy guns. Mainers should vote “yes” on Question 3.