Bush resisted calls for a suspension of shipments to the reserve in the past. When his 2004 presidential opponent, Sen. John Kerry, D-Mass., suggested the same idea during the campaign, Bush called it "playing politics."

On Tuesday, Bush said the nation's 685-million barrel petroleum reserve had enough fuel to guard against any major supply disruption over the next few months.

The president said Democrats in the past have urged higher taxes on fuel and price caps to control fuel expenses, but he said neither approach works. Instead, he called for increased conservation, an expansion of domestic production and increased use of alternative fuels such as ethanol.

David Friedman of the Union of Concerned Scientists said an even more effective move would be to require that vehicles sold in the United States get higher gas mileage.

"The fundamental problem is that the fuel economy of cars and trucks is a disgrace and the world is just consuming too much oil and gasoline," Friedman said.

The EPA said it will consider fuel waivers on a case-by-case basis if gasoline supply problems become apparent, which could result in price spikes or shortages of cleaner summer-blend gasoline.

EPA spokesman John Millett said the waivers would not adversely impact air quality because they are only for 20 days, although states can request extensions.

Refiners, meanwhile, said that most of the change to summer-blend gasoline already has been completed and waivers may not be needed ¡ª and might even be counterproductive in some cases.

"You're going to have to be careful that you're not upsetting a plan that already is in the last stage of implementation," said Bob Slaughter, president of National Petrochemical Refiners Association.