The speculation that the dissent to the opinion upholding the Affordable Care Act was originally the majority opinion and that Chief Justice Roberts changed his mind has inspired this counter-scenario from Orin Kerr at Volokh Conspiracy:
The Justices voted at conference and there were five votes to uphold the mandate on the tax argument and at least five votes to strike down or modify the medicaid expansion. The first group is Roberts plus the liberals, and the second group is Roberts plus the conservatives. Roberts is the swing vote in this case and this is the biggest case of his time on the Court, so he quite naturally assigns the opinion to himself. Roberts doesn’t know how many votes his opinion will get, and he tries to write in a way that might persuade some unlikely votes to join him. Maybe Justice Kennedy will change sides and make the case 6-3, which would avoid the dreaded 5-4 vote. Or maybe he can get some liberal votes to join the section blocking the medicaid expansion.
To write the opinion, Roberts needs to cover a lot of ground — anti-injunction act, tax power, medicaid expansion, etc. Roberts also writes on the Commerce Clause issue, even though it’s not needed to reach the result. Why include that section? Perhaps Roberts thinks that his middle-ground opinion that includes a section agreeing with the mandate challengers on the Commerce Clause might pick up Kennedy’s vote. Or maybe Roberts just wants to weigh in on the most high-profile legal issue of the year, which he happens to care a lot about. He’s also the fifth vote on the Commerce Clause issue and everyone else is writing on it, so he wants to make his views known. Either way, he writes a proposed majority opinion covering all the major issues.
After Chief Roberts circulates his majority opinion, the conservative dissenters decide to write a joint opinion in response. Why a joint opinion? It took Roberts a while to circulate his proposed majority opinion, so the time pressure is particularly intense on the dissenters. The dissenters have a lot to issues to cover and very little time in which to say it, and making it a joint effort allows them to pool resources. They divide the pieces with different Justices working on different issues. The result is a 65 page opinion that is a bit of a patchwork, with different parts by different Justices having different lengths and some portions not really necessary (like severability) included. Some parts may have been drafted before the Roberts opinion circulated, which might explain why parts are duplicative of the Roberts opinion. But the joint dissent is a genuine joint effort, which might explain why parts of the opinion sound like they were written by Justice Scalia and yet Justice Kennedy announced the dissent from the bench.
At the same time the conservative dissenters are writing their response to Roberts from the right, Justice Ginsburg does the same from the left. Ginsburg’s opinion ends up responding mostly to the Chief’s opinion and not the joint dissent, as many have noted. But that wasn’t because Roberts initially was on the other side. Rather, it’s because when Ginsburg was drafting her opinion, the Chief Justice’s opinion was the only one that had circulated. At that point it wasn’t clear how many votes Roberts would get, so Justice Ginsburg treated it as a potential majority opinion even though in retrospect Roberts ended up only writing for himself on the high-profile question of the Commerce Clause.