If the federal government can require people to purchase health insurance, what else can it force them to do? More to the point, what can't the government compel citizens to do? Those questions have been the toughest ones for the Obama administration's lawyers to answer in court appearances around the country over the past six months. And they are likely to emerge again if, as expected, the Supreme Court, as early as Monday, agrees to be the final arbiter of the challenge to President Obama's signature health care initiative. The case focuses on whether Congress overstepped its constitutional authority in enacting parts of the law. Lower courts have reached divergent conclusions. Even judges in lower courts who ultimately voted to uphold the law have homed in on the question of the limits of government power, at times flummoxing Justice Department lawyers.