U.S. House Speaker Nancy Pelosi favors the idea of a program to stimulate car sales through consumer vouchers, her spokesman said, appearing to boost the chances of a bill being pushed heavily by Detroit auto makers.

But the bill has already come under fire from free-trade advocates and some foreign auto makers who criticize a provision precluding vouchers from going toward cars built overseas.

The bill, introduced Tuesday by Rep. Betty Sutton, D-Ohio, would provide on-the-spot vouchers between $3,000 to $7,500 to consumers who trade in older vehicles for new, more fuel-efficient cars and trucks. The size of the vouchers would vary, depending on the fuel economy of the car being purchased.

The older vehicles, required to have been built at least eight years ago, would be scrapped and their parts recycled, while the new vehicles would have to meet a certain fuel economy standard - 27 mpg on highways for cars, 24 mpg for light trucks, Sutton said. Consumers could also opt to receive a $3,000 voucher toward mass-transit fares.

Sutton said the intent is to stimulate slumping auto sales while also reducing car pollution.

Auto-industry allies unsuccessfully pushed for a similar program, dubbed "cash for clunkers," to be included in the recently enacted economic stimulus plan. But supporters hope that the recent success of a program in Germany - which they say boosted February car sales in that country by double digits - will give the idea added momentum.

"The speaker is supportive of this concept and looks forward to reviewing the details of Rep. Sutton's legislation," Drew Hammill, a spokesman for Pelosi, D-Calif., said in an email.

But Sutton's bill - strongly pushed by Ford Motor Co. (F) and backed by the United Auto Workers - would preclude vouchers from being used on some of the most fuel-efficient cars and trucks, including the Toyota (TM) Prius.

The vouchers would also be limited to cars assembled in North America. "I do not think there should be an expectation of American taxpayers (that) we have to use our tax dollars to incent Americans to buy foreign automobiles," said Rep. Candice Miller, R-Mich., a co-sponsor of the Sutton bill.

At least two foreign-based auto makers, Toyota Motor Corp. and Mazda Motor Corp. (7261.TO), have raised concerns about the bill.

Barbara Nocera, Mazda's director of government and public affairs, said the bill violates WTO and Nafta agreements.

"American auto dealers who sell imported international brands, their American employees, and the American communities that benefit from their taxes would be unfairly disadvantaged under the Sutton bill," Nocera said in a statement.

A Toyota spokeswoman said Toyota officials share those free-trade concerns.

"We'd prefer a fleet-modernization program that fairly treats all manufacturers," the Alliance of Automobile Manufacturers, which represents both U.S. and foreign auto makers, said in a statement. "When it comes to getting cleaner, more fuel-efficient vehicles off showroom floors and onto the roads, as an alliance, we believe that a program that equally applies to all manufacturers does the most good toward stimulating vehicle sales."

-By Josh Mitchell, Dow Jones Newswires; 202-862-6637; joshua.mitchell@dowjones.com