Although it’s widely believed that “Obamacare” is here to stay, one lawsuit is threatening to undo President Obama’s landmark health care bill.

“A challenge filed by the Pacific Legal Foundation contends that the Affordable Care Act is unconstitutional because the bill originated in the Senate, not the House. Under the Origination Clause of the Constitution, all bills raising revenue must begin in the House,” the Washington Times notes.

You may recall in June 2012 when the Supreme Court ruled on “Obamacare” that Chief Justice John Roberts defined the bill as a tax, not a mandate. This, according to the Times, is where PFL attorneys saw their opening.

“The court there quite explicitly says, ‘This is not a law passed under the Commerce Clause; this is just a tax,’” foundation attorney Timothy Sandefur said recently. “Well, then the Origination Clause ought to apply. The courts should not be out there carving in new exceptions to the Origination Clause.”

The Times explains the details:

The Justice Department filed a motion to dismiss the challenge in November, arguing that the high court has considered only eight Origination Clause cases in its history and “has never invalidated an act of Congress on that basis.”

The U.S. District Court for the District of Columbia is expected to rule on the Justice Department’s motion “any day now,” said Pacific Legal Foundation attorney Paul J. Beard.

The challenge citing the Origination Clause isn’t the only lawsuit against Obamacare, but it is the only one that has the potential to wipe out the entire act in one fell swoop. Other claims, notably the freedom-of-religion cases dealing with the birth control requirement, nibble at the fringes but would leave the law largely intact.

In their brief, attorneys for the Justice Department argue that the bill originated as House Resolution 3590, which was then called the Service Members Home Ownership Act. After passing the House, the bill was stripped in a process known as “gut and amend” and replaced entirely with the contents of what became the Patient Protection and Affordable Care Act.

Though unorthodox, the government motion argues that using H.R. 3590 as a “shell bill” is not unconstitutional.

“This commonplace procedure satisfied the Origination Clause,” said the brief. “It makes no difference that the Senate amendments to H.R. 3590 were expansive. The Senate may amend a House bill in any way it deems advisable, even by amending it with a total substitute, without running afoul of the Origination Clause.”

The brief notes several cases where shell bills have been upheld by courts.

“[B]ut foundation attorneys counter that those rulings involved the Senate substitution of one revenue-raising bill for another,” the Times notes.

The DOJ also points out that the court has allowed revenue bills to originate in the Senate provided “the money raised was incidental to the bill’s mission.”

“Here, by contrast, it is undisputed that H.R. 3590 was not originally a bill for raising revenue,” said the Pacific Legal Foundation lawsuit. “Unlike in the prior cases, the Senate’s gut-and-amend procedure made H.R. 3590 for the first time into a bill for raising revenue. The precedents the government cites are therefore inapplicable.”

The point of “Obamacare” is to “improve the nation’s health care system,” and it does that “through a series of interrelated provisions, many, if not most, of which have nothing to do with raising revenue,” said the government brief.

But Sandefur disagrees.

“What kinds of taxes are not for raising revenue?” he asked.

Although it’s unclear whether PFL’s lawsuit will scuttle the president’s health care law, one thing is certain: “Obamacare” has at least one more hurdle to clear before final implementation.

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