President Obama's healthcare law faces a series of challenges in three appeals courts starting today as Republican lawyers from 27 states will urge the courts to strike down the law as unconstitutional. In a sign of the high stakes and the partisan divide, one case will feature a rare courtroom clash between the Obama administration's top appellate lawyer and his counterpart from the George W. Bush administration. At issue is whether the government can require virtually all Americans to have health insurance by 2014 or pay a small tax penalty. Democrats said the mandate was needed to make sure that all who could afford to do so paid for medical insurance. Otherwise, they said, freeloaders would force hospitals and taxpayers to pay for them if they were badly injured or came down with crippling diseases. Republicans who opposed the law called the mandate an unprecedented government meddling in private lives. It "would imperil individual liberty" and "sound the death knell" for the Constitution if the government can "compel" people to buy products, Paul D. Clement, the former Bush administration solicitor general, said in a brief last week.