If you want to get a sense of whether the Manchin-Toomey expansion of background checks has any chance of passing the Senate in a future vote, keep an eye on the one-man war Senator Joe Manchin has launched against the NRA.

The NRA had aired an ad targeting Manchin over his advocacy on behalf of expanded background checks, apparently to show that he’d pay a price for his apostasy in the deep red state of West Virginia. Manchin’s response? To hit back hard — and go straight for the NRA’s throat. If it works, Manchin could end up proving that an NRA onslaught can be survived and even defeated, which could put some backbone in other red state Democrats going forward.

Manchin today unveiled a new ad hitting back at the NRA that renews the push for expanded background checks, and casts the NRA leadership, as opposed to its rank and file, as out of touch with West Virginia gun owners. “I believe that we can protect the Second Amendment — and make our communities safer,” Manchin says. “I think most law abiding gun owners agree with me. Call the NRA. And tell them to support criminal background checks.” Watch:

Meanwhile, in an interview on Morning Joe today, Manchin kept it up, making the gun owner’s case for expanded background checks. “As a law abiding gun owner, I’m not going to sell my gun to a stranger,” he said, adding that applying background checks to private sales would reassure those who don’t want to sell their guns to the mentally ill or criminals: “I wanna know who’s gonna buy my gun.”

The NRA ad that kicked off Manchin’s counterattack accused the Senator of turning his back on his own constituents to collaborate with President Obama and New York Mayor Michael Bloomberg. Unlike Senator Mark Pryor — who voted against expanded background checks and has sought to cast the billionaire mayor’s attacks on him over that vote as proof of his red state bona fides — Manchin is directly engaging the substance of this debate, arguing that he, and not the NRA leadership, really speaks for rank and file gun owners in his state.

Roll Call’s David Hawkings had a terrific piece explaining the larger significance of Manchin’s new crusade, which is all about winning over a handful of persuadable Senators in hopes of revisiting Manchin-Toomey later:

[B]efore he can genuinely hope to win over any of the senators seen as persuadable, he’ll have to offer evidence that they, too, can survive the NRA’s attack ad onslaught.

So fighting his own rhetorical war now, with the argument that no law-abiding gun owner’s right to bear arms would be infringed on by more background checks, is what Manchin may view as the best way to help the larger cause.

To be sure, Manchin is not up for reelection until 2018. But this is nonetheless a very interesting experiment, and Manchin’s standing in his state will be very much worth watching. To gun reform advocates, the primary underlying dynamic that must be changed is that too many senators think they will pay a greater price for supporting gun reform than they will for opposing it. They are operating from the beliefs that their constituents are more conservative than they are on gun issues, and that any argument that can be caricatured as anti-Second Amendment probably can’t carry the day and risks tarring you as culturally out of step.

Manchin seems to be trying to prove that the NRA’s attacks can be weathered and the argument with the gun lobby can be won, even in a red state, which might go some way towards undercutting all those assumptions and persuading other senators to rethink them. Interestingly, the NRA — by targeting Manchin so aggressively, despite the Senator’s (previously) sterling gun rights record — may end up being complicit in that outcome.

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