A “cash-for-clunkers” bill seen by domestic auto makers as a potential salve for painfully stagnant new-vehicle sales takes another step forward today by reaching a compromise in the U.S. House of Representatives.

The legislation seeks to modernize the U.S. vehicle fleet by offering consumers vouchers worth up to $4,500 to help pay for a new, more fuel-efficient car or truck when trading in a gas guzzler.

U.S. Rep. John Dingell (D-MI) says the program could not come at a more opportune time. New-vehicle sales so far this year are down 37.3%, according to Ward’s data. The proposed legislation now will be wrapped into an energy bill under discussion by the Committee on Energy and Commerce. If that bill passes the House, it goes to the Senate and then the White House.

“By stimulating consumer demand for new vehicles, this proposal will directly benefit domestic auto workers and automotive manufacturers, which have arguably been hardest hit by the current economic downturn,” Dingell says in statement announcing a compromise on the bill.

The Detroit Three have been clamoring for such legislation in recent months, citing the success of similar programs in other countries. Mike DiGiovanni, chief sales analyst at General Motors Corp., said last week more consumers are thinking of buying, and the time is ripe for a cash-for-clunkers program.

“The last missing link is this auto-sales stimulus package,” he said. “If it strategically comes right now, it is going to be the optimal time to see a really sharp, at least near-term, increase in sales because all of the elements are there.” But when this latest fleet-modernization bill was first introduced in March, it drew criticism from auto makers operating in the U.S. and headquartered elsewhere because vehicles built in the U.S. were set to get bigger vouchers than those built in Canada or Mexico.

Details of the bill are expected later today, so it remained unclear if that element of the program has been fully excised.

This much is clear. For passenger cars, the trade-in vehicle must get less than 18 mpg (13.1 L/100 km) fuel efficiency and the new one must achieve at least 22 mpg (10.7 L/100 km). If the fuel economy of the new car is at least 4 mpg (1.7 km/L) higher than the old vehicle, the voucher is worth $3,500. If the mileage of the new car is at least 10 mpg (4.3 km/L) higher than the old vehicle, the voucher rises to $4,500.

For a voucher on light-duty trucks and SUVs, the trade-in vehicle must get less than 18 mpg and the new one must achieve at least 18 mpg. If the mileage of the new truck or SUV is at least 2 mpg (0.9 km/L) higher than the trade-in, the voucher is worth $3,500. If the mileage of the new truck or SUV is at least 5 mpg (2.1 km/L) higher than the old one, the voucher rises to $4,500.

For larger pickups and vans, or those weighing between 6,000 lbs. (2,721 kg) and 8,500 lbs. (3,855 kg), the trade-in must get less than 18 mpg and the new one must achieve at least 15 mpg (15.7 L/100 km). If the mileage of the new truck is at least 1 mpg (0.4 km/L) higher than the trade-in, the voucher is worth $3,500. If the mileage of the new truck is at least 2 mpg higher than the trade-in, the voucher rises to $4,500.

As far as work trucks weighing from 8,500 lbs. and 10,000 lbs. (4,535 kg), consumers can trade in a pre-2002 vehicle and receive a voucher worth $3,500 for a new work truck in the same or lower weight class. Only a small number of big-truck vouchers will be issued.

Consumers can also “trade down” to receive a $3,500 voucher for trading in an older work truck and purchasing a pickup or van weighing between 6,000 lbs. and 8,500 lbs.

In its current form, the program will last one year and cover 1 million vehicles. It remains unclear how much funding the program might require.