A three-judge panel of the federal appeals court in Atlanta ruled Friday that the health care overhaul law's individual mandate --- requiring Americans to buy health insurance --- is unconstitutional, in one of the largest legal challenges to the Obama administration's signature legislation. The ruling moves the argument a step closer to the U.S. Supreme Court.

It is the second federal appellate court to rule on the law; the first decision, by a Cincinnati court, upheld the measure. It is also the highest-profile ruling on a challenge to the law by 26 states including Georgia.

Gov. Nathan Deal and state Attorney General Sam Olens praised the ruling and said Georgia will continue to work to have the whole law declared unconstitutional.

"Today's ruling recognizes the core principles of our federalist system and reminds an over-reaching federal government that the Constitution applies to it, too, " Deal and Olens said in a joint statement.

Deal and Olens disagreed with parts of the decision, saying the entire law should have been thrown out.

"But this much is certain: Federal health care reform is on life support, and this case will be decided by the Supreme Court of the United States, " Deal and Olens said in their statement. "Today is a huge step toward victory, but it is also a day that emphasizes the importance of the work ahead."

In its 2-1 ruling, the 11th Circuit Court of Appeals panel sided with a federal district court in Florida ruling that the individual mandate is not authorized under the Constitution's Commerce clause, which allows the federal government to regulate interstate commerce.

The appellate court did overturn part of the lower court's ruling by declaring the rest of the law could continue to stand without the individual mandate.

"This economic mandate represents a wholly novel and potentially unbounded assertion of congressional authority: the ability to compel Americans to purchase an expensive health insurance product they have elected not to buy, and to make them re-purchase that insurance product every month for their entire lives, " the panel majority wrote in the 304-page decision.

The law requires most Americans to purchase health insurance in 2014. Those who do not buy coverage will be subject to a fine.

The individual mandate quickly became the most controversial part of the Affordable Care Act after it became law in March 2010. It has been at the heart of 26 federal court challenges.

The White House swiftly denounced the appeals court's ruling. It argued the legislative branch was using a "quintessential" power --- its constitutional ability to regulate interstate commerce, including the health care industry --- when it passed the law. Administration officials said they are confident the ruling will not stand. The Justice Department can ask the full 11th Circuit to review the panel's ruling and will likely appeal to the Supreme Court.

Chief Judge Joel Dubina, an appointee of President George H.W. Bush, and Circuit Judge Frank Hull, appointed by President Clinton, ruled against the individual mandate. Circuit Judge Stanley Marcus, a Clinton appointee, dissented, accusing the court of ignoring "many years of Commerce Clause doctrine developed by the Supreme Court."

Michael Carvin, one of the attorneys who argued for the plaintiffs in the case, said a Democratic-appointed judge ruling against the law "exemplifies that this is a serious issue that is not some kind of partisan policy attack on Obama's health care law, " but "a serious constitutional challenge to an unprecedented act of Congress."

The Supreme Court is expected to take up the case in its next term, which begins this October and ends in June 2012.

Alabama Attorney General Luther Strange called it a "monumental case" for individual liberty. And Texas Attorney General Greg Abbott declared: "Obamacare is closer to an end."

A separate legal ruling Friday also buoyed critics of the law. The Ohio Supreme Court appeared to clear the way for voters there to decide whether to reject parts of the U.S. health law in November.

The federal appeals court in San Francisco, meanwhile, ruled that a former California lawmaker and a legal foundation could not file another challenge on the overhaul.

Staff writer Carrie Teegardin, the Washington Post and Associated Press contributed to this article.