WASHINGTON -- When Senate Minority Leader Mitch McConnell (R-Ky.) filed a brief backing a court case challenging the constitutionality of health care reform, it was hailed as an important gesture of united GOP opposition to the president's signature domestic achievement. Notably absent from the list of signatories, however, were the names of nine of McConnell's Republican colleagues.

The Kentucky Republican filed the brief last week in federal court in Florida, arguing that the individual mandate portion of the Patient Protection and Affordable Care Act (PPACA) is unconstitutional because it gives Congress too much power to regulate citizens' activities. Thirty-one fellow Senate GOPers joined him. The rest did not.

"Where, as in this case with respect to the PPACA's Individual Mandate, Congress legislates without authority, it damages its institutional legitimacy and precipitates divisive federalism conflicts like the instant litigation," argues the senators in the brief. "The long term harms that the PPACA may do to our governmental institutions and constitutional architecture are at least as important as are the specific consequences of the PPACA."

Explanations for the abstainers range from the obvious to the speculative. Among the list are three members who will not be in the body next year. Sen. George Voinovich (R-Ohio) is retiring and has shown little willingness to get too political in his final weeks and months. The same could be said of Sen. Judd Gregg (R-N.H.), though when asked why he hasn't joined the rest of his GOP colleagues, the New Hampshire Republican's office was comically mute.

"We are not going to be able to talk about that," said Otto Heck, a spokesman for the senator. Why not? "We just aren't going to talk about that."

Sen. Bob Bennett (R-Utah), who was defeated in his efforts to win the Republican nomination this cycle, has also abstained from the suit. But removal from office isn't likely the driving reason. The Utah Republican authored a bill in 2007 that was based around the individual mandate -- a legislative venture that proved, in the end, to be a key contributor to his downfall.

That bill, the Healthy Americans Act, was co-sponsored by two other Republican senators who have, to date, declined to participate in the suit challenging the Affordable Care Act's constitutionality: Sens. Lamar Alexander (R-Tenn.) and Lindsey Graham (R-S.C.). Neither Alexander nor Graham's office returned a request for comment.

That leaves four Republican senators without obvious explanations for abstaining from the health care suit. Senator Richard Lugar (R-Ind.) would seem like a natural signatory, owing to his upcoming re-election bid in 2012. But the Indiana Republican also is one of the president's few GOP allies in the Senate. And when asked for an explanation of his position, his spokesperson said he did, in fact, believe the bill was unconstitutional, but wanted the issue dealt with either on the state level or through legislative mechanisms.

"Under Indiana law, Senator Lugar took the lead in asking the state attorney general to file suit against the health care law," said Mark Helmke, senior advisor to Lugar. "Since there is already legal proceedings going on by Indiana's AG, the Senator decided to focus on legislation."

Sen. Lisa Murkowski's (R-AK) office, meanwhile, insisted that she too shares the philosophical objections to the health care reform law that spurred 32 Republican senators to sign the amicus brief. But the Alaska Republican has been busy waging a write-in campaign to keep hold of her Senate seat and apparently lacked the time or bandwidth to add her name to the list.

"[S]he believes that the mandate that all Americans buy health insurance is unconstitutional and supports repealing the law and replacing it with sensible alternatives that are widely supported, such as the ability to buy insurance across state lines, implementing medical malpractice reform and reimbursing for quality of service and not quantity of services," said Michael Brumas, Murkowski's communications director.

While Murkowski's election bid was preventing her from joining the amicus brief, Sen. Scott Brown's (R-Mass.) forthcoming campaign should have compelled him to do just that. The Massachusetts Republican ran for office in 2009 largely on a promise to stop the health care law in its tracks. And with his re-election approaching, remaining in the good graces of the Tea Party would seem like a real priority.

Instead, the Massachusetts Republican has chosen to play to their devotion to state rights. Rather than signing on to the suit, he has collaborated with Sen. Ron Wyden (D-Ore.) -- the same legislative partner that Bennett chose -- to figure out a way around the individual mandate. Last week, the two introduced legislation that would effectively allow states to develop their own health care system (including one without the mandate) should they be able to guarantee certain standards of coverage and costs.

That leaves, Sen. Jeff Sessions (R-Ala.) as the one Republican senator whose decision to not sign on to the amicus brief seems utterly inexplicable. His office did not return a request for comment.

Just as curious as Session's non-response to the suit are some of those Republicans who have jumped on board. Sen. Chuck Grassley (R-Iowa) was a co-sponsor of the Wyden-Bennett bill. During a June 2009 appearance on "Fox News Sunday," moreover, he was asked what, exactly, was wrong with the individual mandate.

"There isn't anything wrong with it," he replied, "except some people look at it as an infringement upon individual freedom. But when it comes to states requiring it for automobile insurance, the principle then ought to lie the same way for health insurance, because everybody has some health insurance costs, and if you aren't insured, there's no free lunch. Somebody else is paying for it."

Why he and other former individual mandate supporters -- namely, Sen. Orrin Hatch (R-Utah) -- suddenly have deep constitutional concerns with the provision isn't entirely clear. Both have stressed that they want state government in charge of such decisions. A skeptic, however, might note that Hatch is up for re-election in 2012, as is Olympia Snowe (R-ME), a GOP moderate who dramatically joined McConnell's suit late last week.

"As I asserted during the debate on this legislation, the individual mandate has no place in a health care reform bill unless and until affordable health insurance is available for all Americans," said Snowe.

UPDATE, 1:46 p.m.: Grassley's office sent over a statement, citing the president to explain his opposition to the individual mandate:

The debate over the individual mandate has been rigorous. President Obama staunchly opposed it when he was a candidate for President. Now he supports it. A lot of concerns he raised on the campaign trail are very legitimate. They became more clear as the debate unfolded last year and the public had a chance to weigh in. Enforcement is a major issue. Also remember that the insurance companies lobbied for the mandate because it drives billions of tax dollars in new subsidies directly to them. Also, the non-partisan Congressional Research Service said in a memo that using the Commerce Clause to impose a penalty to enforce the individual mandate is a novel approach. CRS stated: "This is a novel issue: whether Congress can use its Commerce Clause authority to require a person to buy a good or a service and whether this type of required participation can be considered economic activity." Now the novel approach is being tested in court.