The two presidential contenders weighed in quickly.
President Obama called the ruling "a victory for people all over this country. ... What we won't do — what the country can't afford to do — is refight the political battles of two years ago or go back to the way things were."
Presumptive GOP nominee Mitt Romney declared: "What the court did not do on its last day in session, I will do on my first day as president, and that is, I will act to repeal ObamaCare."
The decision seemed to stun most Court observers.
"It was a really surprising decision because no one was expecting this to hinge on whether this is an exercise of Congress' taxing power," said Tom Christina, a health care attorney at Ogletree Deakins.
Roberts' Rule Of Order
Chief Justice John Roberts, writing for the majority, said that the ObamaCare mandate to buy insurance was not constitutional under the Commerce Clause because that gives Congress the power to regulate commerce, not compel it. Roberts also said the "necessary and proper" clause didn't apply. Even if the mandate is "necessary" to carry out parts of ObamaCare, compelling commerce isn't "proper," he argued.
But Roberts did rule that the mandate or, more specifically, the penalty for not buying insurance could be construed as a tax. He argued it didn't matter if calling the mandate a tax was the likeliest interpretation, only that it was a "fairly possible" one.
He cited several ways in which the mandate looks like a tax. For example, he noted that it is paid to the Treasury by "taxpayers" when they file their tax returns. He also noted that the penalty was determined by factors such as taxable income, joint filing status and number of dependents. He further explained that previous Court rulings allowed that things not called a "tax" but rather a "penalty" were justified under Congress' power to tax.
The four dissenting conservative justices rejected Roberts' reasoning, arguing that a tax and penalty are "mutually exclusive" and that the entire law should be thrown out.
Other conservatives scoffed as well.
"The Court stretched the statute," said Robert Alt, senior legal fellow at the Heritage Foundation. "Calling this a tax is not what Congress or the president intended."