*'Cash for Clunkers'

* Obama, auto manufacturers support voucher plan (Adds wrangling over Guantanamo prison provisions)

By Jeremy Pelofsky

WASHINGTON, June 4 (Reuters) - U.S. congressional Democrats are considering fast-tracking legislation to boost auto sales by offering Americans vouchers to trade in older, less fuel-efficient vehicles for ones that get better mileage, a congressional aide said on Thursday.

The legislation, known as "cash for clunkers," would be added to a pending war funding measure in Congress, but "no final decisions have been made" despite wide support for the voucher program, said Nadeam Elshami, a spokesman for Democratic House Speaker Nancy Pelosi.

The war funding bill is a must-pass spending bill that is largely for U.S. forces operating in Afghanistan and Iraq. Adding the car program would speed the voucher measure's enactment into law.

Lawmakers have been trying to find new ways to spur auto sales as two of the big three U.S. automakers are in bankruptcy protection, Chrysler LLC and General Motors GM.N. President Barack Obama has urged Congress to approve a voucher program.

The Senate proposal would provide up to $4,500 in vouchers to car buyers who participate in the program.

New cars must get at least 22 miles per gallon, while sport utilities and pickups -- the biggest sellers for U.S. manufacturers -- must achieve at least 18 mpg. Vehicles priced at over $45,000 are not eligible.