The lawsuit filed by automakers against states that have adopted California’s greenhouse-gas restrictions on vehicles may be dismissed as early as tomorrow (Wed.) afternoon.

When the Supreme Court ruling in Mass. vs. EPA was announced, the judge handling the automakers’ lawsuit — Judge William K. Sessions III of the U.S. District Court of Vermont — summoned the lawyers in the case to his chambers for a discussion in light of the ruling. Check out the court’s schedule for tomorrow morning.

If the suit is dismissed, it will be, in practical terms, a much more significant consequence of Mass. v. EPA than any regulations EPA is likely to pass during Bush’s presidency. It will set the stage for California and eight other states — possibly soon more — to implement tough greenhouse-gas regulations on vehicles.

That would be the automakers’ worst nightmare: CAFE standards or no CAFE standards, state air-pollution standards would force them to start making fuel-efficient vehicles.

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Some background:

As most Grist readers know, in 2002 California passed what’s known as its Pavley law (AB 1493, named after California Assemblywoman Fran Pavley), instructing the California Air Resource Board (CARB) to develop and implement greenhouse-gas restrictions on new vehicles, beginning with the 2009 model year. In 2004, CARB approved such regulations, which would require around a 30% reduction of greenhouse gas emissions by vehicles by 2016. Those regulations have since been adopted by eight other states (two more states are considering them).

Automakers responded by filing a half-dozen lawsuits in several of those states.

They made several claims, but the primary claim was that by restricting GHG emissions specifically for vehicles, California was in effect setting its own fuel-economy standards. By law, no state is permitted to set fuel-economy standards more strict than those set by the federal Dept. of Transportation. (By contrast, the Clean Air Act does allow states to set air pollution standards more strict than federal standards, as long as they get a waiver from the EPA.)

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In Jan. 2007, a federal judge put the case on hold, pending a ruling in Mass vs. EPA, wherein the Supreme Court would address just that claim, among others. Whatever it decided would apply to lower courts as well.

As it turn out, the ruling in Mass. v. EPA utterly destroyed the claim (as reported by NYT’s Felicity Barringer). The crucial bit is on p. 29 of the majority ruling (PDF):

EPA finally argues that it cannot regulate carbon dioxide emissions from motor vehicles because doing so would require it to tighten mileage standards, a job (according to EPA) that Congress has assigned to DOT. See 68 Fed. Reg. 52929. But that DOT sets mileage standards in no way licenses EPA to shirk its environmental responsibilities. EPA has been charged with protecting the public’s “health” and “welfare,” 42 U. S. C. §7521(a)(1), a statutory obligation wholly independent of DOT’s mandate to promote energy efficiency. See Energy Policy and Conservation Act, §2(5), 89 Stat. 874, 42 U. S. C. §6201(5). The two obligations may overlap, but there is no reason to think the two agencies cannot both administer their obligations and yet avoid inconsistency.

In other words: air-pollution standards, including standards regarding GHGs, are independent of fuel-economy standards.

This leaves the automaker lawsuits against the states with Pavley regulations essentially baseless. Which is why the judge is likely to toss them.

Once that happens, expect to see California quickly get its CAA waiver from the EPA. Once that happens, expect to see Pavley regulations keep spreading, and possibly get boosted by ambitious states.

Never mind CAFE standards. This is what’s finally going to force automakers to start making fuel-efficient vehicles. Thanks, SCOTUS!