Only three months after the Newtown massacre brought the nation together in mourning, the issue of preventing gun violence is breaking down along partisan lines.

This week, what was once considered the least controversial gun control measure — closing the private-sale exception to the instant background check law — barely made it out of the Judiciary Committee on a 10-8 party-line vote, with all Republicans in favor of keeping the gun show loophole wide open. This is disappointing for anyone who hoped that responsible gun owners and other concerned citizens could come to agreement on what polls show most Americans consider good policy.

Background checks have been required since 1998, and they are used by licensed gun dealers to screen out unqualified buyers with criminal or mental health records. But while a gun buyer has to go through a check inside L.L. Bean or the Kittery Trading Post, no such check is required for a sale that takes place in the parking lot. Seven states require universal background checks, but the rest, like Maine, provide an easy way for a prohibited person to buy a gun.

Despite this, the checks work. Thousands of sales are stopped every year, even with this porous enforcement.

Still, the National Rifle Association and its political allies argue that universal checks are unnecessary because criminals will find a way around them. The same argument could be made against every law on the books. Homicide laws don’t prevent all murders, but no one argues that we would be safer without them.

Maine’s gun-owning tradition should not prevent Maine’s senators from doing the right thing and supporting this bill. This is not a new restriction on gun ownership; it simply creates a mechanism to enforce the current law. Requiring a private buyer and seller to visit a licensed dealer for a background check is a small incovenience. But it could prevent a tragedy that could not be undone.

Sens. Susan Collins and Angus King should vote to close the private-sale loophole and show that protecting public safety doesn’t have to be a partisan affair.