Every Senator who is refusing to support expanded background checks — Republican or Democrat — needs to be asked a simple question: Do you support the current background system, or do you see it as an infringement on the rights of the law-abiding?

Every one of them will answer with a Yes, because they are taking refuge behind the idea that the current law needs to be strengthened in various ways but not expanded. Once they are on record confirming they don’t view the current system as a threat to Constitutional rights, the arguments against expanding it dissolve into incoherence.

The Senators who are threatening to filibuster Obama’s gun proposals (Rand Paul, Marco Rubio, Ted Cruz, and Mike Lee) have said that they will “oppose any legislation that infringes on the American people’s right to bear arms, or on their ability to exercise this right without being subjected to any additional government surveillance.”

But even libertarian Tea Party chieftain Rand Paul has allowed that current background checks “work.” And on the Sunday shows yesterday, other Republican Senators, such as Lindsey Graham and Jeff Flake, said they could support improving the current system through better data sharing by states on the mentally ill and other such moves, while opposing expanding checks to private sales. As Steve Benen notes, this means “leaving the massive gun show loophole in place.”

That’s true, and I’d add one other point: It means these Senators view the current background check law as constitutional. Which means there is no logical way to argue that expanding background checks is an infringement on Constitutional rights. Here’s why: The compromise background check expansion being negotiated would simply build on current law, which requires gun dealers (who would conduct the checks on private sales) to keep records on those sales; it explicitly forbids the creation of a national registry; and it requires the feds to destroy info collected on legit gun transfers within 24 hours. None of this — none of it — would change. If the current law is not an infringement on constitutional rights, then neither is an expanded one.

To be fair, in their Sunday appearances, Graham and Flake didn’t argue against the proposal on Constitutional grounds, as the four Tea Party Senators have. But they both dissembled about the plan, with Graham falsely suggesting a father-son gun transfer could be targeted (the compromise proposal under discussion exempts family members), and Flake lamenting new “paperwork requirements” (which would be identical to current ones).

All these Senators should be pressed on whether they support the law requiring private citizens who purchase guns from federally licensed dealers to undergo a check. When they confirm that they do, they need to be pressed on why applying that same system to private sales — in which private citizens who buy guns from another private citizen must undergo a check — is objectionable, particularly since for the buyers, nothing changes, and since these Senators themselves concede we need to do a better job preventing criminals and the mentally from buying guns.

Senators holding out against expanded checks need to be pushed hard on this stuff. This is an important proposal, with American lives potentially at stake.

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