6.29.12 In a defining moment in U.S. Supreme Court history, Chief Justice John Roberts joined the court’s liberal bloc Thursday to announce a 5-4 decision upholding the most hotly debated provision of President Barack Obama’s 2010 health care overhaul. To do so, the Roberts majority framed the requirement for all Americans to purchase health insurance, along with an IRS penalty for not complying, as a tax, not the argument the administration preferred but enough to leave in place the linchpin of the insurance market changes.

Yet even as Roberts, a Republican appointee, crossed the court’s ideological divide on the individual mandate, he also led seven justices in striking down perhaps a wider-reaching component of the law that would have compelled the 50 states to oversee a significant expansion of the joint state-federal Medicaid insurance program for the poor. The expanded framework still stands, but participation will be up to individual states.