Led by Florida, 26 states have challenged President Obama's Affordable Care Act, passed two years ago without the vote of a single Republican. Next week, the US Supreme Court will hear three days of oral arguments on the case — the first time that's happened in 45 years. But laws like this, that raise constitutional issues and intimately affect tens of millions of people, don't come around very often. Lower courts have been sharply divided on the principal question: can the federal government punish Americans who don't buy health insurance? Can the government mandate that Americans buy health insurance? What's at stake for the legacies of Chief Justice Roberts and President Obama in the midst of an election year?