It’s widely assumed that if Congress fails to pass a clean energy bill, the EPA will step in with regulations on CO2 under the Clean Air Act. The Supreme Court ruled in 2007′s Mass. v EPA that it must do so if it finds CO2 to be a dangerous air pollutant — and sure enough, the agency sent the White House its final endangerment finding Monday. EPA regulations now appear inevitable and unstoppable. But don’t be so sure.

The threat of EPA CO2 regs is a thorn in the side of fossil-fueled legislators and one of the few points of leverage green Dems have. It has hovered over congressional climate negotiations, bringing recalcitrant lawmakers to the table. It’s generally agreed by both sides that regulatory emission restrictions would be worse for power companies than legislative restrictions; a recent Wall Street Journal story covered several utilities lobbying for legislation on that basis. EPA regs would be “more arbitrary, more expensive, and more uncertain for investors and the industry than a reasonable, market-based legislative solution like cap and trade,” said Exelon head John Rowe. Some enviros have gone so far as to claim that it would be preferable for the weak legislation in Congress to fail so that tougher EPA regs could take its place. (A dangerously wrong notion, IMO.)

Is it true, though, that EPA regulations are inevitable and unstoppable? It might seem so, given the stark clarity of the Supreme Court’s ruling. But never underestimate the plasticity of congressional procedure or the willingness of conservatives to use any means necessary to protect their corporate constituents.

I put the question to a senior Senate legislative aide a while back: Is there really nothing Republicans and conservative Dems can do to stop the EPA? He smiled ruefully and told me to look into what happened to CAFE standards in the mid-’90s. This Congressional briefing paper (PDF) tells the story:

In October 1993, less than one year after taking office, the Clinton administration issued its Climate Change Action Plan, and this included a process that was to be co-chaired by the White House National Economic Council, Office of Science and Technology Policy and the Office of Environmental Policy to reduce greenhouse gas emissions from motor vehicles. In April 1994, it published an advanced notice of proposed rulemaking to develop fuel economy standards for light trucks for model years 1998-2006. Seven months later, Republicans won control of Congress and promptly began to attach “riders” on annual appropriations bills to prevent funding for administration activity to develop or implement new fuel economy rules for light trucks. These riders blocking progress on fuel economy improvements remained in place until President Bush took office.

Could the same thing happen to EPA regs that happened to CAFE regs under Clinton? Well, Sen. Lisa Murkowski (R-Alaska) has already tried once, back in September. Her amendment was poorly written and she ultimately backed down without forcing a vote on it. But as the aide told me, it would be possible for a more adept legislator to write a more carefully tailored amendment that would block only the stationary-source regulations and leave the (more popular) vehicle regulations untouched. Obviously Republicans don’t control Congress now, and unless the most catastrophic predictions play out, won’t in 2010 either. But hostility to EPA regulations on power plants cuts across party lines. And remember, what’s needed here isn’t 60 votes against the EPA regs per se — just 60 senators who think passing an appropriations bill is more important than standing up for the EPA. The thing about appropriations bills is that they really need to pass or parts of the federal government go unfunded. There’s enormous pressure; that’s why members of Congress are fond of attaching riders to them.

EPA opponents will have plenty of opportunities to build a coalition, as E&E reports (sub rqd):

Sen. John Thune (R-S.D.), who cosponsored Murkowski’s amendment, said there would be “extremely dangerous consequences” if the administration is allowed to “unilaterally” regulate greenhouse gas emissions and that the cost of gasoline, food and manufactured goods would skyrocket. He said the EPA regulations should be delayed until Congress has had a chance for a full and open debate on the issue.

“This issue will be back,” Thune vowed. “Senator Murkowski will bring it back; I will bring it back.”

Sen. Lamar Alexander (R-Tenn.), the ranking member of the Interior Appropriations Subcommittee, welcomed the senators’ opportunities to air their grievances on the floor.

“I’m glad it was debated and I think Senators Murkowski and Thune were right to bring it up and it got them the chance to make the point,” he said, adding that the point “will be made over and over again.”

Over and over again, whee! In terms of raw numbers, there are probably more than 60 senators hostile to EPA regs. The question is whether some core number of coal-state Dems can be kept in line in the name of party discipline. You know how Senate Dems love party discipline.

What happens if an appropriations bill with an EPA-blocking rider comes to a vote? The only option for green Dems would be to filibuster. There are certainly legislators who seem willing to do so. In a conversation with National Journal last week, Sen. John Kerry said this:

I’m going to make this as clear as I can: I don’t think anybody is going to wind up repealing [EPA CO2 regulations] because there’s filibuster-proof capacity to prevent that from happening. … I’ll personally stand on the Senate floor day and night to prevent that from happening, and there are plenty of procedural ways in which to do that. So that’s not going to happen. I don’t see any scenario in which that does, and there are plenty of people who would stand there with me. This is not a solo effort by any sense of the imagination. As I’ve said, there is a clear number of votes that would not allow that to happen, assuming we’re moving in good faith down the road.

This is tough talk. And there’s plenty of precedent for blocking appropriations bills (see this section of Filibuster: obstruction and lawmaking in the U.S. Senate, by Greg Wawro and Eric Schickler). Dems famously filibustered a defense appropriations bill with a rider that would have opened the Arctic Refuge to drilling.

But Kerry’s talking about mustering 40 liberal senators to block a much-needed bill on behalf of a policy that both the White House and EPA have spent the last year badmouthing and that most “centrist” senators oppose. That will be tricky political terrain, to say the least.

If there’s a sufficiently large bloc of senators motivated to block the EPA, they’ll probably find some way to block it. But the point here is not so much to try to predict what might happen. It’s just to say that EPA regulations of CO2 are not “inevitable.” Nothing in politics is inevitable; nothing’s a sure thing; everything’s a risk; everything’s a fight. Those who would abandon legislation in Congress in favor of EPA regs run at least some risk of consigning the U.S. to years without any restrictions on CO2 emissions.