If the Obama administration had any doubt that its signature healthcare law faces a severe challenge in court, it was erased soon after Chief Judge Joel Dubina opened the proceedings here. "I can't find any case like this," Dubina said. "If we uphold this, are there any limits" on the power of the federal government? Judge Stanley Marcus chimed in: "I can't find any case" in the past, he said, where the courts upheld "telling a private person they are compelled to purchase a product in the open market.... Is there anything that suggests Congress can do this?" After nearly three hours of argument Wednesday, the three-judge panel of the 11th Circuit Court of Appeals seemed prepared to declare at least part of last year's law unconstitutional. The law's requirement that nearly everyone buy health insurance by 2014 is the question at the heart of the constitutional challenge. The argument that the mandate exceeds Congress' power initially was waved aside by many legal commentators, but it has now sharply divided the federal courts.