Forget hydrogen. The car of the future has an extension cord and a great big laptop battery.

The next evolution of the automobile will be plug-in hybrids that get their juice from a household electrical outlet. They'll start rolling into showrooms within in 18 months. Experts say plug-in hybrids could account for about 20 percent of vehicle sales within a decade – and half of all sales by 2050.

"It all boils down to the three ways electricity is better than gasoline," says Felix Kramer of Cal Cars, a plug-in advocacy group. "It's cleaner, it's cheaper and it's domestic."

Advocates say plug-in hybrids are the best chance to address global warming and wean the nation from oil. Consumers remain unsure about electric vehicles. Ethanol's a shaky proposition because of the food-for-fuel debate. And it'll be decades before hydrogen is a viable option. That, advocates say, leaves plug-ins as the best option. They'll go up to 40 miles on a charge; but they'll also have a gas engine to keep you going beyond that at 80 to 100 mpg or more.

People have been converting conventional hybrids to plug-ins for years, but the auto industry has been slow to catch on. Now the big automakers and start-ups like Fisker Automotive are scrambling to build them despite questions about their cost and long-term reliability. Those are just two of the issues that automakers, battery manufacturers and utility companies will discuss next week at the international Plug-In 2008 conference in San Jose.

"The discussion is no longer one of 'if,' but of 'when' and 'how,'" says Chelesa Sexton, executive director of the advocacy group Plug-In America. "This has moved beyond the grass-roots level into the policy and business arenas."

It all starts in 2010. General Motors promises to have the Chevrolet Volt rolling into showrooms by then. Toyota says it will roll out a small fleet of plug-in Prius hybrids to see how they do. Volkswagen has similar plans for its plug-in Golf. And Fisker hopes to have a few dozen pricey Karma sedans in driveways within 18 months. Ford and others are moving more slowly, aiming for 2012 and beyond.

Automakers know plug-in hybrids are their best shot at meeting tightening federal fuel-economy regulations, and California's zero-emissions-vehicle mandate requires them to put nearly 60,000 of them on the road in six years. They're also responding to a seismic change in the market as record-high gas prices have consumers, fed-up with paying through the nose for gasoline, joining environmentalists to demand fuel-efficient cars.

"For the longest time, this was seen as a crunchy environmental California movement," Sexton says. "It never was, but now there's a broad coalition of people sitting at the same table to demand these cars. There's a collective frustration with the status quo."

Critics note that most of our electricity is generated by coal or natural gas and say plug-ins don't reduce carbon dioxide, they just move it around.

Mark Duvall of the Electric Power Research Institute says they're wrong. His research shows widespread adoption of plug-in hybrids could cut greenhouse gas emissions by more than 450 million metric tons annually by 2050. That's the equivalent of removing 82.5 million gasoline vehicles from the road. "There's significant CO2 reduction with plug-in hybrids over conventional vehicles and hybrids, and that reduction increases over time," he says.

Duvall's research and a study by the Pacific Northwest National Laboratory suggest that the grid could easily supply as many as 168 million plug-in vehicles.

"We can handle as many plug-in hybrids as the auto industry wants to provide and people want to drive," he says. "The supply of electricity is almost limitless."

All those plug-ins would cut petroleum consumption from 20.6 million barrels a day to 16 or 17 million. But the lithium-ion batteries that will store that electricity remain the cars' Achilles heel.

The long-term reliability of lithium-ion batteries remains unknown, and by some estimates they cost as much as $15,000. That'll make selling plug-ins at a price most people can afford a tough proposition until the cars are made in volume – and the cost of batteries comes down. GM says it doesn't expect to turn a profit on the $40,000 Volt anytime soon.

Sales undoubtedly will start off slowly. Analysts don't expect GM to sell more than 30,000 Volts annually for the first couple of years. Other automakers will see similar sales figures until the cost of batteries comes down.

"We're looking at small volumes initially," says Mike Omotoso of J.D. Power & Associates. "But we could see critical mass by 2015."

Advocates say politicians and policymakers can help by creating tax breaks to make it easier for consumers to buy the cars and automakers to build them. Such incentives – coupled with perks like carpool-lane access – helped hybrids gain a foothold, they say, and could do the same for plug-ins.

The Department of Energy has handed out more than $60 million since 2006 to advance hybrid and battery technology and hopes to disburse another $62.3 million by the end of next year.

Both Barack Obama and John McCain have hailed plug-in hybrids in general – and the Volt in particular – in recent weeks and promised to spur development of such cars if elected. And Sen. Lamar Alexander, R-Tennessee, has called for Washington to go further by launching a "New Manhattan Project" that would include getting plug-in hybrids on the road in large numbers.

"We have the plug," he says. "The cars are coming. All we need is the cord."