Now the waiting begins–or resumes. After nearly seven hours watching opposing sides duke it out in a Beltway hearing room this week, the EPA will settle down to deciding (again) if California should be allowed to set its own standards for auto emissions.

During the hearing, one group was using Twitter to pass around an online petition supporting the required EPA waiver. They weren’t too late. EPA will continue accepting public comment until April 6. EPA spokesman Cathy Milbourn says “We will review all of the comments, with a decision to follow.” No further timeline for that decision has been made public, however.

Meanwhile, the Detroit News is reporting today that California’s top air regulator may be ready to compromise on a new national standard that would obviate the need for a special waiver.

In case you need a quick review, the issue is whether the tailpipe emissions standards passed into law by California several years ago–the so-called Pavley regulations–can actually be enforced. The Pavley standards are more stringent than the current federal standard and the state is leaning heavily on them to attain its greenhouse gas targets under the Global Warming Solutions Act of 2006 (AB 32). But the waiver was denied under the Bush administration.

Thirteen other states are lined up to enact the California standard if they get a green light from EPA. The auto industry has long argued that this will create a “patchwork” of regulations across the nation, and the ensuing complications of compliance would place an onerous burden on the industry and push up prices for car buyers.

Supporters of the California standard, like Jim Kliesch of the Union of Concerned Scientists, say that automakers already have the technology and can easily comply. Kliesch conceded that consolidating the most efficient technology into one car would add–he figures–about $700 to the cost. But he says the same technology would recoup $1,800 in fuel savings over the life of the car.

Mark Cooper of the Consumer Federation of America pointed to an apparent disconnect in the car maket. He referred to a survey in which half the respondents said they wanted their next car to get at least 30 MPG–but Cooper said only 2% of models currently on the market deliver that.

And so, the argument goes, that if car makers would just follow the market toward cleaner, more fuel-efficient cars, it would actually help them recover from a financial abyss that threatens to topple them.

At the end of the day, the EPA has to make its decision based on three criteria, says David Doniger of the NRDC. To be valid, the California standard must be:

1. Equally strict or more stringent than the federal standard,

2. Needed to meet “compelling and extraordinary conditions,” and

3. Technologically and economically feasible.

Hmm. It seems like you could make a solid case for checking off numbers 1 and 2 but what’s “economically feasible” is a potential tripwire, especially with General Motors teetering on the brink of bankruptcy. Much of it will come down to whether the Obama administration buys into the “patchwork” argument. It’ll be at least another month before we know.