On the eve of an historic Senate health care reform vote, more than half of the Republican state attorneys general have organized to provide a legal analysis of the constitutionality of a controversial provision in the Senate bill and to explore potential legal challenges.

The provision in question is the result of a deal Senate Democratic leaders struck with Nebraska Sen. Ben Nelson to win his critical vote. Under the agreement — dubbed the “Nebraska Compromise” or the “Cornhusker Kickback” by GOP critics — Nebraska will get an exemption from the state share of Medicaid expansion, a carve out that is expected to cost the federal government $100 million over 10 years.

South Carolina GOP Attorney General Henry McMaster, who is leading the effort, said the intent is to determine whether Congress had the power to allot funds to Nebraska differently from other states.

“I think it’s unprecedented to have this use of federal power,” said McMaster. “We heard about it, read about it, decided something needed to be done about it.”

McMaster began to organize the effort after South Carolina GOP Sens. Lindsey Graham and Jim DeMint wrote a letter to him Monday asking him to investigate the matter.

“We have serious concerns about this Nebraska compromise as it results in special treatment for only one state in the nation at the expense of the other 49,” wrote Graham and DeMint. “While South Carolina has to struggle to come up with hundreds of millions of dollars to comply with the massive new Medicaid mandate, Nebraska does not have to come up with a single dollar.”

McMaster told POLITICO Wednesday that 10 of the 19 GOP state attorneys general have signed on, with many of them taking part in a Tuesday conference call on the issue, and he expected more to join.

Democrats immediately derided the effort as a naked partisan play designed to thrust the GOP attorneys general — a handful of whom, including McMaster, are running for governor in 2010 — into the spotlight.

“It’s a political stunt,” House Majority Whip James Clyburn told POLITICO in a statement. “The politics of states' rights are wearing thin with South Carolinians. McMaster and his partisan counterparts who are running for governor in their respective states are simply trying to raise their profiles. It did not work with the [American] Recovery [and [Reinvestment] Act funding, and he is just as wrong in this instance.”

"Republican governors and candidates are making a serious mistake if they think simply opposing everything President [Barack] Obama does makes for a winning strategy in 2010,” said Emily DeRose, a spokeswoman for the Democratic Governors Association. “Americans want their governors to be focused on creating jobs in their states, not lobbing partisan bombs in D.C.”

Certainly, it was hard to overlook the distinctly nonlegalistic approach various attorneys generals took Wednesday toward the provision.

McMaster, Michigan Attorney General Mike Cox and Pennsylvania Attorney General Tom Corbett have publicly highlighted their actions, with all three taking to their Twitter accounts to highlight their sharp opposition to the Nelson deal.

“The Nebraska Healthcare Deal Comes at MI taxpayer's expense. I am astounded by this sweetheart deal and will oppose it,” wrote Cox.

“The deal cut w/ Ben Nelson amounts to nothing more than vote buying at taxpayers expense. And it must be challenged,” wrote McMaster on his feed.

“Attorney General Corbett analyzing constitutionality of 'Nebraska Compromise' in health care bill,” tweeted Corbett, in one of the more measured accounts.

While acknowledging that his initiative had attracted the attention of a slew of politically ambitious AGs, McMaster denied there was any political motivation.

“No, it’s just a question of the law,” said McMaster. “I think even [President] Lyndon Johnson would be surprised by what goes on in the Senate today.”

Colorado GOP Attorney General John Suthers also dismissed the Democratic criticism, telling POLITICO: “How do Democratic attorneys general look citizens in the eye and say, ‘It’s OK for you to pay Nebraska Medicaid expenses so we can round up enough votes on health care?’”

Washington state Attorney General Rob McKenna said the group was still trying to determine what potential legal recourse existed — and that one option would be a lawsuit if the Senate bill was signed into law.

“One of the reasons we have checks and balances is to raise questions about what the majority is doing,” said McKenna in an interview. “We’re taking a second look at [the Nelson agreement] because we think it’s what the Constitution was designed to prevent.”

With the Senate set to pass its version of the health care bill before breaking for Christmas, Alabama GOP Attorney General Troy King predicted that the probe would wrap up in the next few days.

“I think we’re moving rapidly,” he told POLITICO. “We’ve got to move quickly. We don’t have a choice.”