They're hot, they're tough and they can carry an entire family of six along with the family dog. Even Cadillac, that purveyor of elite luxury vehicles, is making them. But despite their popularity, sport utility vehicles may be headed for extinction.

If the environmental lobby is successful in its efforts to raise federal fuel efficiency standards on cars and trucks, automakers will have no choice but to stop making SUVs — much the way station wagons went the way of the dinosaur when standards were first enacted in 1975.

"It's not clear to me how you are going to make any real SUV get 40 miles [of gas] per gallon," said H. Sterling Burnett, a senior fellow at the Center for Policy Analysis in Dallas.

Advocates for raising the standards say that even though fuel efficiency standards were implemented, more cars are polluting the air today and the United States has become more dependent on foreign oil and more vulnerable to hostile powers abroad. The government, therefore, should encourage companies to design SUVs capable of running on something other than oil or at least make them more fuel efficient.

Two bills pushing this goal are awaiting Senate debate. Sen. John McCain, R-Ariz., has introduced a bill that would start in 2007 and force manufacturers to build cars and light trucks that would get 36 miles to the gallon by 2016. Sen. John Kerry, D-Mass., supports a bill that would force the same vehicles to get 35 miles to the gallon by 2013.

Today's current Corporate Average Fuel Economy (CAFE) standards, which were first imposed 27 years ago, require cars to get 27.5 miles to the gallon and light trucks to get 20.7 miles to the gallon. SUVs are kept to the light truck standard, which many critics are calling a "loophole," because they are used primarily as passenger vehicles.  

"Closing the SUV loophole would also prevent 200 million tons of carbon dioxide, the main greenhouse gas, from entering the atmosphere each year," claimed Rep. Jay Inslee, D-Wash., last May.

Opponents, however, say the fuel economy standards have failed by encouraging more driving since it becomes less expensive to drive, the greater the mileage per gallon.

CAFE standards, they also argue, have forced manufacturers to build smaller cars that result in greater road fatalities and have not led to less dependence on foreign oil.

"If people want to drive fuel efficient automobiles, there are plenty of them on the market already," said Charli Coon, senior fellow for energy and environment at the Heritage Foundation. "The whole purpose of [CAFE standards] was to reduce our dependence on oil and the number of cars on the road. But we've basically doubled that. There just isn't a silver bullet for reducing oil consumption."

Raising the CAFE standards even more would just exacerbate the problem, said Burnett. Just like the station wagon, which was phased out because it could not be made to passenger car standards, the SUV will be no more and consumers will be forced to drive lightweight cars that fuel-reduction opponents say do not provide as much security.

"I think it's a bankrupt policy that kills citizens. It's already killing people and will continue killing people as long as it's on the books," Burnett said.

He pointed to a Harvard University/Brookings Institution study that found that for every 100 pounds shaved off a vehicle, between 440 and 780 additional people were killed in auto accidents, or 2,200 to 3,900 lives lost per model year. This study compares to other findings by the National Traffic Safety Commission.

But consumer and environmental advocates like Joan Claybrook, president of Public Citizen, says that's just one way to twist the numbers to fit the argument.

"That's poppycock," she said Monday. Most of the changes made after the last time the standards were raised in 1985 were technological, rather than weight-based, she said.

The debate, expected to heat up over the next few weeks as the Senate wrangles with a massive energy bill that will include everything from oil drilling in Alaska to measures to keep a lid on greenhouse gases, will most likely play out along the powerful fault lines of Washington's heaviest special interest hitters: automakers, unions and environmentalists.

The latter says it's about time the "Big Three" automakers — Ford, General Motors and DaimerChrysler — start designing cars that help the environment rather than fighting it so aggressively.

"They haven't done anything in 17 years, you would think they would give a little," Claybrook said.

But Burnett says the big loser will be the consumer, for the day such fuel efficiency standards are passed, the death knell for the SUV will be sounded.

"I really don't know how you can keep them," he said. "What this is really about is automobile choice in the end. Consumers choosing what cars they want to drive. People are already voting with their pocketbooks."