Updated

After sitting for months on the sidelines, Missouri's Democratic attorney general has thrown his support behind a landmark multi-state lawsuit challenging the federal health care overhaul.

Chris Koster -- the top attorney in the state which last year passed the first-in-the-nation ballot measure against the law -- on Monday filed what's known as a friend-of-the-court brief in the case before the 11th Circuit Court of Appeals. The action involved a bit of hedging. In filing the brief, he did not formally join the other 26 states in the suit. He also expressed support for the health care law's goal of expanding health coverage and argued that most of the language should be upheld.

But Koster said the so-called individual mandate requiring most Americans to buy health insurance runs afoul of the Constitution, as well as his state's referendum, and should be cleaved from the law.

The mandate, he said, "would imbue Congress with police powers rejected by the Founding Fathers and never before permitted by the Supreme Court."

Koster, as other critics of the requirement have argued, said the administration's reliance on the Commerce Clause regulating economic activity is not valid. With the mandate, he said the U.S. government is trying to regulate an individual's decision not to purchase a product -- in this case, health insurance. In other words, the administration is trying to regulate economic inactivity.

"Within the health care arena, the power to penalize one's decision not to purchase health insurance is indistinguishable from granting Congress the power to penalize individuals for not obtaining an annual check-up or prostate exam, for not vaccinating one's children, or for not maintaining a specific body-mass," Koster wrote.

Missouri's Lt. Gov. Peter Kinder, a Republican who has led the charge against the health care law in his state, applauded Koster for his legal argument.

But he needled the attorney general for waiting so long to make it, and for endorsing the high-profile suit led by Florida as opposed to one he filed in Missouri.

"After 13 months of silence on the sidelines, my friend Attorney General Koster has joined the battle after a fashion," Kinder told Fox News.

He said the decision to link up with the Florida suit, and not Missouri's, is "puzzling," and suggested Koster was making a late-in-the-game political maneuver.

"Ask anybody who knows him, my friend Chris Koster has finally developed political antenna," he said.

Koster, while arguing against the mandate, urged the appeals court to rein in the decision of a lower-court federal judge who not only ruled the mandate unconstitutional but invalidated the entire health care law along with it.

Koster wrote in his brief that such a ruling "does not require the entirety of (the health care law) be struck down."

"This court should restrict its ruling to the individual mandate and dependent provisions," he wrote.