Derek Thompson rounds up the tough individual mandate questions asked by the Justices today. He calls Justice Kennedy's worries, reproduced below, "perhaps the most important paragraph of the morning." What Kennedy said:

The reason this is concerning, is because it requires the individual to do an affirmative act. In the law of torts our tradition, our law, has been that you don't have the duty to rescue someone if that person is in danger. The blind man is walking in front of a car and you do not have a duty to stop him absent some relation between you. And there is some severe moral criticisms of that rule, but that's generally the rule. And here the government is saying that the Federal Government has a duty to tell the individual citizen that it must act, and that is different from what we have in previous cases and that changes the relationship of the Federal Government to the individual in the very fundamental way.

Sarah Kliff decodes some legal jargon:

“Limiting principle” is a phrase that came up a lot in the Supreme Court Tuesday morning - 15 times, according to the transcript. It’s a legal concept you’ll probably hear a lot about in this afternoon’s analysis. When courts review a new application of Congress’s constitutional authority, they historically wanted to see the government articulate a clear limit to those powers - they look for, in legal jargon, a “limiting principle.”

Lyle Dennison puts Kennedy's comments under a microscope:

If Justice Anthony M. Kennedy can locate a limiting principle in the federal government’s defense of the new individual health insurance mandate, or can think of one on his own, the mandate may well survive.  If he does, he may take Chief Justice John G. Roberts, Jr., and a majority along with him.  But if he does not, the mandate is gone.  That is where Tuesday’s argument wound up — with Kennedy, after first displaying a very deep skepticism, leaving the impression that he might yet be the mandate’s savior.

So does Orin Kerr:

Reading the tea leaves, it sounds like Justice Kennedy accepts the basic framework of the challengers that mandates are different and especially troubling. Instead of saying that mandates are therefore banned, however, Justice Kennedy would require the government to show some special circumstances justifying the mandate in each case. The answered question in this case is whether the special economics of the health care market justifies the mandate here.

Adam Serwer can't believe that Obamacare's defenders were so unprepared: