A cash-for-clunkers program that would provide government vouchers to consumers to trade in their old cars faced an uncertain fate Thursday.

Lawmakers had attached the program to a must-pass bill to finance the wars in Iraq and Afghanistan, but talks on the broader legislation collapsed Thursday over unrelated issues.

The program that had been included in the war-funding bill closely resembled a bill passed by the U.S. House this week to provide vouchers of up to $4,500 to consumers to trade in their old, gas-guzzling cars for new and more-efficient models.

Under the House bill, trade-ins would have to get no higher than 18 mpg and have been built in 1984 or after. A $3,500 voucher would then be issued for the purchase or lease of a vehicle that got at least 22 mpg. The voucher would increase to $4,500 if the new vehicle was 10 mpg higher in fuel economy than the trade-in. Vouchers would be limited to vehicles under $45,000.

The standards would be looser for light-duty trucks.

President Obama has called for Congress to pass a so-called cash-for-clunkers program to stem a near-historic slide in auto sales.

"We cannot wait any longer to pass this legislation," Rep. John Dingell, D-Mich., said. "Every day we put it off, auto sales are depressed further."

The House bill, sponsored by Rep. Betty Sutton, D-Ohio, and aggressively pushed by U.S. auto makers, faced opposition from several senators who say it wouldn't go far enough to reduce car emissions of greenhouse gases.

The critics, including Sen. Dianne Feinstein, D-Calif., and Sen. Susan Collins, R-Maine, contend that the House bill could provide vouchers toward the purchase of high-polluting Hummers and SUVs.

"Drivers in a tough economy need more incentives for fuel efficiency, not subsidies for inefficient vehicles that will cost more in the long run," Collins and Feinstein wrote in an opinion column published in Thursday's Wall Street Journal.

They pushed an alternative bill that they said would result in nearly a third more oil savings than the House bill. Their bill would set stricter fuel-economy requirements than the House version and allow vouchers to go toward used vehicles.

Proponents point to programs in Germany and other European countries that helped lift auto sales in those countries by double-digit margins.

The bill passed by the U.S. House would last a year and cost about $4 billion. One question that remains is where the funding would come from. Supporters hoped to include about a $1 billion for the program in the war-funding bill, with more money coming later, possibly from the economic-stimulus plan put in place earlier this year. The plan would subsidize an estimated 1 million vehicles, lawmakers estimate.

Democrats like House Speaker Nancy Pelosi and Rep. Henry Waxman of California and Rep. Edward J. Markey of Massachusetts came out strongly for the House bill, blunting criticism that the bill was the pet project of the auto industry.

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