The hint of the outcome during the first day of oral argument (on the impact of the Anti-Injunction Act on the Court’s jurisdiction to hear the ACA case)
I think there was a clue to Roberts’ thinking during the first day of argument—during the argument on the applicability of the Anti-Injunction Act, an obscure “jurisdictional” statute, which precludes courts from ruling on the constitutionality of a federal tax until after the statute becomes effective and the tax actually is due. Roberts really indicated during that argument that he was interested in finding a way to rule that the penalty was a tax for purposes of Congress’s taxing power but not a tax for purposes of the Anti-Injunction Act. That would remove the issue of whether the mandate was within Congress’s Commerce power, since if it is a tax, the mandate and penalty/tax are within Congress’s taxing power. And then the only issue would be whether this violated the Fifth Amendment’s due process clause (liberty! broccoli!).
Late in the argument on the mandate issue the next day, under questioning by Sotomayor, the challengers’ lawyer, Paul Clement, conceded that under its taxing power, Congress could do pretty much the same thing as it did under what Congress thought its Commerce power allowed it to do. That effectively killed the due process (liberty! broccoli!) argument, since for purposes of that argument, it made no difference which Congress’s powers authorized it to enact the mandate and the related penalty.