One of the key GOP Senators that Dems are quietly trying to win the support of on expanded background checks is John McCain. His backing would make it significantly easier to pass the proposal through the Senate with real bipartisan support — which in turn would make it more likely that House Republicans would allow a vote on it.

So it seems highly significant that McCain previously supported closing the private seller loophole — so much so that he actually cut ads in support of the idea, claiming: “With rights come responsibilities.”

The ad shows that this is a position that even conservative Republicans should theoretically have no ideological difficulties embracing.

In the October 2000 ad, McCain — a reliable ally of the NRA and decorated war veteran who had gained national prominence with his failed insurgent presidential candidacy that year — suggested closing the private seller loophole would significantly reduce gun violence.

“Convicted felons have been able to buy and sell thousands of guns at gun shows because of a loophole in the law,” McCain says in the spots, which were done for a gun control group. “Many were later used in crimes.”

McCain calls for “requiring criminal background checks by unlicensed dealers at gun shows,” adding: “I believe law abiding citizens have the right to own guns. But with rights come responsibilities.” Here’s the spot, which was unearthed  by a Democrat:

In the ads, McCain is pushing for two state ballot initiatives — one in Colorado, and one in Oregon — that would require background checks on private sales. But McCain saw the state initiatives as a response to a problem on the federal level, i.e., the private seller loophole that existed at the time and still exists today.

Indeed, in an interview at the time, McCain explicitly linked his efforts on behalf of these state initiatives to the need to close the loophole in federal law, and vowed to push for closure of the loophole in Congress the next year.

In that interview, McCain also said that his views on guns had changed in response to violence at the time, including the shooting at Columbine High School:

“I do believe my view has evolved,” he said. “It’s appropriate to do so in light of some of the terrible tragedies that have befallen our nation.”

In theory, at least, this should make it harder for McCain to oppose the proposal to close the loophole in the wake of the horrific Newtown shooting.

What’s particularly interesting here is that McCain was staking out what was then the moderate middle ground. At the time, the left pole of the gun control debate was defined partly by opposition to the idea of a gun ownership right, with some arguing that it only existed on Constitutional grounds in the context of militia membership. McCain’s position put him squarely in the middle between gun control groups and gun rights forces.

Since then, the Supreme Court decision in District of Columbia v. Heller — which struck down D.C.’s handgun ban — upheld a Constitutional right to gun ownership for traditionally lawful purposes. With that Supreme Court precedent set, it should theoretically be even easier for Republicans to accept the middle ground position of universal background checks, which don’t threaten rights that are now enshrined by the Court. But neither McCain nor any other Republican Senator (except for Mark Kirk) has so far proven willing to take the step McCain did back in 2000, underscoring how far to the right the debate remains, even in the wake of the massacre of 20 children.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.