Today begins an extraordinary trio of days of oral argument at the Supreme Court concerning the constitutionality of Obamacare. The issue today is a preliminary one, not the main act. That doesn’t mean it is unimportant.

The issue for the day is whether the Anti-Injunction Act, which prohibits a lawsuit to challenge a tax before the tax has been paid, prevents the Supreme Court from considering the constitutionality of Obamacare until 2015, when individuals could first sue for a refund on any fines imposed for failure to comply with the individual mandate.

This is a binary issue: Either the Anti-Injunction Act applies (and the only other issue the Supreme Court would decide would be the constitutionality of the Medicaid provision) or it doesn’t apply, in which case there is no escaping ruling on the merits of the individual mandate.

No issue is a “slam dunk” before the Supreme Court, but in this case both the administration and the challengers say the Anti-Injunction Act doesn’t apply because for statutory purposes the penalty is not a tax. (The administration nevertheless is clinging to its position that it is not a tax for constitutional purposes, despite the president’s own words). The Supreme Court therefore has separately appointed a lawyer to argue that the Anti-Injunction Act does in fact bar the challenges until a penalty is paid.

Robert Alt of the Heritage Foundation writes:

Notwithstanding the rare unanimity of the parties on this point, some speculate that the Court may rely on the AIA to avoid answering the controversial question of the mandate’s lawfulness, an outcome that may particularly appeal to Chief Justice Roberts, who generally favors judicial minimalism. This is, of course, little more than tea-leaf reading, but there are reasons to believe that the leaves do not portend this outcome. First and foremost, the legal arguments in support of the AIA bar are weak, and the Court (particularly the Chief Justice) is unlikely to bend the law to avoid a controversial ruling. Second, as a practical matter, a ruling that the AIA prevents a judgment now only delays the inevitable and creates uncertainty in the interim.

Since Chief Justice John Roberts is the most “minimalistic” of the so-called conservative judges, we should pay careful attention to his questions. Is he seeking a way not to reach the meta-issue of the individual mandate?

In addition, it will be noteworthy if President Obama’s own words come up. Not only is the “tax” issue relevant to the Anti-Injunction Act, but it is also a substantive defense for the administration on the constitutionality of the individual mandate. We’ll see if the former Chicago Law School constitutional law professor blew an argument for his side.

The real show comes tomorrow when the individual mandate is the topic of argument and on Wednesday when the Supreme Court handles the most more intricate question of whether the rest of the statute, or parts of the rest, must also be struck down if the individual mandate is struck down. On this latter point, after reading a number of the briefs and discussing the matter with some amici brief lawyers, I am convinced that the severability arguments are much more challenging for the government than many initially believed, given the way the individual mandate impacts a variety of provisions. But I’ll save that for Wednesday.

Think of today as the appetizer. We may hear a few zingers, but the real battle begins tomorrow.