Requiring background checks on firearm purchases at gun stores does not violate the Second Amendment. Neither would requiring background checks on firearms purchases at gun shows. Yet while the former requirement is federal law, the latter is not.

A bill from Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., aims to close the illogical “gun show loophole” — and to mandate background checks for Internet gun sales.

And when the Senate voted by a 68-31 margin Thursday to open formal debate on that proposal, thereby averting a filibuster threat, it was a victory for common sense.

It was also a welcome signal that the National Rifle Association’s Capitol Hill clout, though still considerable, has limits.

That Senate bill, if it had been law late last year, would not have saved the 20 first-graders and six adults gunned down at Sandy Hook Elementary in Newtown, Conn. But that doesn’t make the legislation unworthy of passage.

Neither does the NRA’s opposition to it.

The NRA “grades” lawmakers on the federal and state levels.

NRA chief lobbyist Chris Cox sent a letter to senators on Wednesday, informing them that those grades could be adversely affected if they backed the Manchin-Toomey bill — or even merely backed averting a filibuster against it. Who elected him?

Sixteen Republicans, including South Carolina Sen. Lindsey Graham, defied that NRA warning Thursday. (Twenty-nine Republicans, including South Carolina’s Tim Scott, voted for the filibuster option.)

Unfortunately, some of the senators who helped move the bill forward to debate actually oppose its ultimate passage. That group also includes Sen. Graham, who explained in a written statement Thursday:

“Filibustering right now means senators get a free pass. Some politicians’ dream scenario is one where they don’t actually have to vote for or against a proposal but can tell each side they were with them all along. Again, let’s make senators vote.”

Sen. Graham hailed his “own legislation, supported by the NRA, which I want to bring forward in the Senate.”

He introduced that bill in reaction to a chilling incident here two months ago, when Alice Boland allegedly aimed a handgun at school officials outside Ashley Hall and pulled the trigger several times.

Fortunately, though the gun was loaded, there was no round in the chamber.

According to authorities, Ms. Boland bought that firearm at a gun store in Walterboro, passing a background check there despite having pleaded not guilty by reason of insanity to 2005 charges of threatening to kill President George W. Bush.

Sen. Graham says his bill would strengthen the background-check system to prevent people with mental problems from buying guns.

But citing concerns about facilitating federal gun registration, he doesn’t want background checks at gun shows.

That’s a “slippery slope” argument we can’t buy.

After all, if background checks are needed to keep convicted felons, fugitives from justice and the mentally ill from buying firearms at gun stores, they are needed for the same crucial purpose at gun shows.

The NRA’s knee-jerk resistance to reasonable proposals to regulate firearms, along with its persisting influence in Washington, doesn’t just undermine public safety.

It undermines respect for the Second Amendment right “to keep and bear arms.”

The bill that dodged the filibuster bullet Thursday still has a long way to go. Even if it eventually passes the Senate, it would face tough odds in the House. And the NRA has apparently succeeded in thwarting congressional efforts to restore the 1994-2004 assault-weapons ban and to pass new limits on high-capacity ammunition magazines.

Still, the NRA didn’t get its way in our nation’s halls of power Thursday.

And that was a refreshing change from gun-policy business as usual.