A question of commerce
In one of his first questions, Justice Kennedy asked if the government could force someone into commerce in order to regulate them under the commerce clause." That could be the distinction, Barker explains, between this case and Wickard vs. Filburn, which is the case the government is relying on for the mandate.
That New Deal case tested the powers of Congress to regulate the grain market and keep grain prices in check by limiting the amount of grain that could be produced by any one farmer. Filburn, an Ohio farmer, contended that he could grow as much grain as he wanted because he was using the excess for his family. In that case the court ruled that activity constitutes commerce even if the activity doesn't have an impact on interstate commerce.
For PPACA the administration is arguing that inactivity—not purchasing health insurance—falls under the same ruling because people who don't purchase health insurance make it more difficult and costly for those who do. Solicitor General David Verrilli argued that "what is being regulated is the method of financing the purchase of healthcare. That itself is an economic activity with substantial effects on interstate commerce."
In reviewing the transcript, Verrilli seemed to struggle with this argument as Justice Scalia and Chief Justice Roberts asked if the government could require the purchase of cell phones and even broccoli under the commerce clause.