'Critics of this plan ridiculed our notion that we should use part of the money to modernize the entire fleet of federal vehicles to take advantage of state of the art fuel efficiency. This is what they call pork,' Obama said at a speech at the U.S. Energy Department in Washington. 'You know the truth. It will not only save the government significant money over time, it will not only create manufacturing jobs for folks who are making these cars, it will set a standard for private industry to match.'

Obama's defense was the first time he's addressed it in public. It comes as automakers have been cutting thousands of jobs amid a sharp decline in sales.

'When you hear these attacks deriding something of such obvious importance as this, you have to ask yourself -- are these folks serious? Is it any wonder that we haven't had a real energy policy in this country?' Obama said.

McConnell's office didn't immediately return a request for comment.

The $600 million could fund the purchase of about 30,000 vehicles at a time when auto sales have collapsed -- falling 37 percent last month. It would require the government to buy vehicles that were at least 10 percent more efficient than those being replaced.

January's monthly sales rate total was the lowest since June 1982. Chrysler LLC's U.S. sales dropped 55 percent, while General Motors Corp.'s sales fell 49 percent. Ford Motor Co.'s sales fell 40 percent, according to Autodata Corp.

Late Wednesday, Sens. Tom Harkin, D-Iowa, and Debbie Stabenow, D-Lansing, said they will introduce an amendment dubbed the 'Sell Fuel Efficient Cars Amendment' that would provide a rebate of $10,000 to buyers who trade in a car more than 10 years old for a new car assembled in the United States. The amendment provides $16 billion which would cover more than 1.5 million purchases. The program would end once the funds ran out, but no later than Sept. 30, 2010.

The Harkin amendment may be voted on today, but he may not seek a vote.

Some automakers criticized the measure. 'The Harkin Buy American amendment clearly violates the WTO & NAFTA trade agreements,' said Barbara Nocera, director of government and public affairs at Mazda North American operations. 'And, furthermore, it smacks of the type of protectionism that led to the Great Depression.'

The Association of International Automobile Manufacturers, a trade association representing Toyota Motor Corp., Honda Motor Co. and Nissan Motor Co among others, also opposes the Harkin Amendment, said Mike Stanton, president of AIAM.

The Alliance of Automobile Manufacturers, which represents Detroit's Big Three and eight other automakers, also said they may oppose the bill.

The Senate bill also doubles to 500,000 the number of plug-in electric vehicles that could qualify for up to a $7,500 tax credit.

The bill would provide $300 million to replace or retrofit diesel engines and use $400 million to help state and local governments buy more efficient alternative-fuel vehicles.

'The recovery bill's $600 million provision to fund the purchase of fuel-efficient vehicles is both good economic policy and good energy policy,' said Ross Eisenbrey, vice president at the Economic Policy Institute, a Washington think tank. 'As the automakers launch new electric vehicles, plug-in hybrids and other high-mileage vehicles, the government can guarantee a market for them even if cheap gasoline dampens the enthusiasm of individual consumers.'

Earlier this week, the Senate approved an $11 billion tax break to allow most consumers to deduct the sales tax on new cars and the interest paid on new car loans on their taxes.

Obama also used the speech to defend efforts to allocate $2 billion for advanced battery research in the stimulus, saying investments 'in technologies that will accelerate the development of innovations like plug-in hybrid vehicles, we'll be making a significant down payment on a cleaner and more energy independent future.'