In a stunning victory for the Obama Administration, the U.S. Supreme Court on Thursday upheld the centerpiece of the nation’s new healthcare law — the so-called individual mandate to buy insurance — as a constitutional exercise of Congress’ taxing authority.

Chief Justice John Roberts Jr., who wrote the Court’s opinion, secured the majority on the tax issue by joining with his more liberal colleagues — justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

The Court also ruled that the Affordable Care Act’s major expansion of the federal-state Medicaid program unconstitutionally coerced the states. The coercion was in the federal government’s threat to withhold all of its existing Medicaid funds to states that failed to participate. But instead of striking down the program expansion, the court held only that the federal government could not withhold those funds. States now can decide whether to participate in the expansion.

Four justices — Antonin Scalia, Anthony Kennedy, Clarence Thomas and Samuel Alito Jr. — would have struck down the entire law.

Roberts opened the morning and final session of the term before a packed courtroom. Outside of the building, a line of visitors stretched from the front plaza across the street to the Library of Congress. A large group of supporters and opponents rallied on the sidewalk.

As the solicitor general of the United States and the other lawyers in that office sat at the lawyers’ table directly below the bench, Roberts began his summary of the healthcare decision by tackling what had been the government’s primary argument in defense of the law: Congress’ authority under the commerce clause.

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