The Obama administration's answer: "It depends."
This is the remarkable answer that an administration lawyer responded to an appellate judge's question. It came during a court hearing on the ObamaCare provision that requires all Americans to buy health insurance. Here is the exchange:
“Let’s go right to what is your most difficult problem,” Judge Laurence H. Silberman, who later voted to uphold the law, told a lawyer at an argument in September before the United States Court of Appeals for the District of Columbia Circuit. “What limiting principle do you articulate?” If Congress may require people to purchase health insurance, he asked, what else can it force them to buy? Where do you draw the line?
Would it be unconstitutional, he asked, to require people to buy broccoli?
“No,” said the lawyer, Beth S. Brinkmann. “It depends.”
She didn't elaborate on what it "depends" on, and was hit with some other examples:
Could people making more than $500,000 be required to buy cars from General Motors to keep it in business?
“I would have to know much more about the empirical findings,” she replied.
Judge Brett M. Kavanaugh, who ended up in dissent, then jumped in. “How about mandatory retirement accounts replacing Social Security?” he asked.
“It would depend,” Ms. Brinkmann replied.
This exchange in found in an excellent New York Times article by Adam Liptak, in which he explains the central issues that the U.S. Supreme Court will take up when it considers that constitutionality of ObamaCare.