The Hill's Sam Baker reports on the most glaring weakness in the legal defense of ObamaCare:

Even in the cases it has won, the administration has failed to answer a key question: If Congress has the power to enforce the insurance mandate, where does that power stop?

It’s known in legal jargon as a “limiting principle.” When courts evaluate a new application of Congress’s constitutional authority, they have historically wanted to see clear limits to those powers.

“The DOJ has to do a better job of answering, ‘What goes beyond your theory of federal power?' " said Ilya Shapiro, a legal scholar at the libertarian Cato Institute who opposes the insurance mandate. “They’ve been asked this in every court and they’ve never satisfied the court, even in the cases they’ve won.”

The 11th Circuit Court of Appeals — the specific case now before the Supreme Court — struck down the insurance mandate partially on the grounds that upholding it would open the door to a flood of regulation.

“Ultimately, the government’s struggle to articulate … limiting principles only reiterates the conclusion we reach today: There are none,” the court said in its ruling.

Baker is correct to identify the open question of what the government can't do if the mandate is constitutional as the strongest argument against the law. The administration has not merely struggled to articulate a limiting principle; it has conceded that under its defense of the individual mandate, there is no limit to the government's power under the Commerce Clause. As the D.C. Circuit Court ruling noted:

The Government concedes the novelty of the mandate and the lack of any doctrinal limiting principles; indeed, at oral argument, the Government could not identify any mandate to purchase a product or service in interstate commerce that would be unconstitutional, at least under the Commerce Clause.

This is a tough question for the administration to answer not merely because it's hard to imagine any limits on Commerce Clause power if the mandate is legal; it's also difficult because the administration has an incentive to avoid preemptively limiting its own power by drawing bright legal lines about what it can and cannot do. Regardless of their party affiliation, administrations tend to spend a lot of time developing legal arguments for why they have the power to do whatever it is they'd like to do; they tend to be somewhat less invested in figuring out how their legal arguments might limit their power to do something they might want to do in the future. 

Does this mean that the Supreme Court is likely to rule against the mandate? Not necessarily. Despite noting the lack of a limiting principle and acknowleding some "discomfort" with the potential for endless regulation under the administration's interpretation of the law, the D.C. Circuit Court ruled in favor of the law