As Barack Obama battled Hillary Rodham Clinton over healthcare during the Democratic presidential primaries of 2008, he was adamant about one thing: Americans, he insisted, should not be required to buy health insurance. "If things were that easy," Mr. Obama told the talk show host Ellen DeGeneres in February of that year, "I could mandate everybody to buy a house, and that would solve the problem of homelessness. It doesn't." Now President Obama may wish he had stuck to those words. On Monday, the Supreme Court agreed to take up a constitutional challenge to his landmark health care bill, and a decision could come in the midst of Mr. Obama's 2012 re-election campaign.