Dodd criticized it, too.

"There are so many loopholes in it, you could drive a Humvee through it," Dodd said of Obama's plan. "If that's what we adopt, you will not get anywhere near the standards we are talking about. It's not going to happen, I promise you."

Obama's campaign says he is trying to reach a middle ground that will set aggressive goals while taking into an account factors that would reduce the rate of progress.

"This debate has been stuck in neutral for 20 years," said Obama spokesman, Bill Burton. "The bipartisan legislation Obama worked to produce has changed the politics of this issue."

Edwards would have no exceptions to reaching the fleet-wide standard of 40 mpg, said his spokesman, Eric Schultz.

This past week, a Senate committee approved a plan that would raise the nationwide fleet fuel economy to an average of 35 mpg by 2020.

Auto executives have said they support an increase in government standards, as long it is determined by experts at the National Highway Traffic Safety Administration.

They appear to have a more sympathetic ear among Republican presidential candidates, including Arizona Sen. John McCain . McCain has voted for some fuel economy increases in the past, but says he would negotiate with automakers on the appropriate increase if he were president.

"I'm not prepared to name a certain number," McCain said in a recent interview with The Associated Press. "I'm hopeful that the marketplace itself and the sale of hybrid cars - the (Toyota) Prius, electric cars - would address this issue in an effective fashion."

Former Massachusetts Gov. Mitt Romney opposes increasing the standards without taking other steps that include increased use of alternatives fuels such as ethanol and biodiesel and the development of hybrid technologies, said his campaign spokesman, Kevin Madden.

Sen. Sam Brownback , R-Kan., has voted against increased standards in the past, but said the threat of US dependence on foreign oil has him so concerned that he would consider changing his position. He said an increase couldn't be so steep that it would endanger the US auto industry while it is at financial risk.

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