"When the price of gasoline goes up ... it's a serious problem we've got to do something about," he said. "We're watching very carefully to make sure people are being treated fairly."

Democrats hoping to take control of the US Congress in November elections have seized on the issue to make a populist argument against big oil companies and Republicans' ties to them.

They are also seeking to tap into public angst over rising gas prices as a way to blunt the White House push to take credit for strong economic numbers overall.

"I know that many of you are anxious right now," Sen Bill Nelson, a Florida Democrat, said in the weekly Democratic radio address. "And Americans are frustrated, and sick and tired of billion-dollar giveaways to the oil companies, while the price of gas goes through the roof," said Nelson.

Bush, a former Texas oilman, has called for the United States to kick its "addiction" to oil, but there is little he can do to bring the cost of gas down in the short term.

In Sacramento, Bush toured the California Fuel Cell Partnership, a consortium of auto makers, energy companies and government agencies supporting the commercialization of vehicles run on hydrogen fuel cells that would emit no pollution and be more efficient than gas-powered cars.

"I strongly believe hydrogen is the fuel of the future," Bush said, "not a foolish dream."

DEMOCRAT SAYS OIL CRISIS COMING

Many experts say it is unlikely fuel-cell vehicles will be ready for widespread use for two or three decades, but Bush pledged to make hydrogen-powered vehicles "a reality for American drivers."

The administration is also funding research into a new generation of plug-in hybrid vehicles that could be recharged in electrical outlets, allowing many drivers to make their daily commute using no gas.

Nelson, in his radio address, supported alternative fuels but said Bush needed to take "more dramatic steps" including conservation and raising the mileage standard for all passenger vehicles to at least 40 miles per gallon.

"The crisis is coming," Nelson said. "And so America must act now, before soaring prices and a dependence on foreign oil puts a chokehold on our economy and military."

Other Democrats also weighed in, with New York Sen. Charles Schumer calling for "reexamination of whether having only four giant oil companies can co-exist with the needs of the American consumer and rational policy on energy."

New York Rep. Louise Slaughter, the top Democrat on the House of Representatives' Rules Committee, has asked for immediate hearings into legislation to reduce gas prices.

Environmentalists welcomed Bush's focus on fuel-cell technology, but Roland Hwang, vehicles policy director for the Natural Resources Defense Council, warned against selling a promising long-term solution as "a quick fix" for political cover.