In a 5–4 decision today, the Supreme Court struck down the Obama Administration EPA’s signature “Mercury and Air Toxic Rule,” which regulates emissions by fossil-fuel-fired power plants. Before regulating, EPA was obligated to decide whether regulation under one the Act’s most burdensome programs was “appropriate and necessary.” EPA interpreted that language to preclude it from considering the costs of regulation—some $10 billion per year, in exchange for $4 million or so in direct benefits. That interpretation, the Court decided, was ludicrous.

The decision may well leave the Obama climate agenda in tatters. Why that is requires a bit of explanation. In the usual case when the Court finds a rule to be unlawful, it vacates the offending action—in other words, deprives it of legal force. But that’s not what the Court did here. Instead, it sent the case back down to the D.C. Circuit for further proceedings, knowing full well that that court will follow its usual practice of “remand without vacatur”—in other words, let the agency fix any flaws in its rule while leaving the rule in place.

This is a very big deal. The centerpiece of the Obama Administration’s climate agenda is EPA’s so-called “Clean Power Plan,” which aims to cut power plants’ carbon-dioxide emissions by around 30 percent and force the phase-out of coal-fired generation. But the statutory authority that EPA claims supports this effort explicitly carves out any regulation of facilities that are already subject to regulations like the Mercury Rule. So if the rule remains in place—as seems likely—then the Clean Power Plan should be dead in the water.

But there’s a more subtle, and perhaps more important, reason to expect trouble ahead for the Clean Power Plan. The Supreme Court’s failure to vacate the Mercury Rule reflects its recognition that the bulk of the rule’s costs—and it was one of the most expensive government regulations ever—has already been borne by industry. So there’s no urgency, at this point, to putting the rule on hold; to the contrary, doing so would be disruptive. But the flip side is that this means utilities and their customers spent tens of billions of dollars complying with a regulation that was always unlawful. One can imagine that the Court won’t be eager for that to happen again. And one can also imagine that the Court’s decision today is a shot across the bow of the D.C. Circuit: when the next billion-dollar rule comes up, and there’s any legal question about EPA’s authority, put the rule on hold so the courts have a chance to do their business. More reading-between-the-lines: if you don’t, we will.

The Clean Power Plan is expected to be finalized in late August, and challengers will ask the D.C. Circuit for a stay just as soon as they can. If that happens, the rule most likely won’t go into effect during the Obama Administration and, depending on the results of the next election, may never go forward.

And that’s why today may be the beginning of the end of the Obama Administration’s climate agenda.