Back in September, we noted that the Environmental Protection Agency (EPA) and Department of Transportation (DOT) were pushing a new scheme under which new cars sold in the U.S. would be given a letter grade assessing their “greenness,” prominently displayed on the window sticker that purchasers see when they visit their local auto dealer. At that time, the scheme had attracted criticism from car dealers, the Alliance of Automobile Manufacturers, a majority of consumers polled by Edmunds.com and even some green types.

Now, it is taking fire from 53 Congressmen on both sides of the aisle.

On Wednesday, a letter bashing the proposal was sent to EPA Administrator Lisa Jackson and Transportation secretary Ray LaHood. In it, the Congressmen point out that the letter-grade label downplays the number of miles per gallon a car gets– a metric the letter’s authors assert consumers are “very familiar” with and which they rely upon– in favor of the letter grade. That letter grade, they argue, is biased in favor of certain kinds of vehicles, and favors electric cars and plug-in hybrids over all other cars– even some that are demonstrably very green, like VW and Audi TDI models, which have recently won “Green Car of the Year” awards.

“The ‘A’ and ‘A+’ categories are reserved for a very narrow range of vehicles, i.e., battery electric vehicles and plug-in hybrids. However, a fuel efficient, clean diesel vehicle would be penalized with a low or mediocre grade. Similarly, most fuel efficient SUVs and pickup trucks would rate no higher than a ‘C+’,” the letter notes.

Some critics of the labeling scheme charge that it may serve as an inadvertent boost to General Motors, since Chevrolet produces the Volt, one of the only cars that would receive one of the highest grades under the proposal.

Others say this is another example of a proposal that is grounded in good intentions, but just does not make much sense in the real world. “A builder who needs a truck to transport lumber isn’t going to be in the market for a Chevy Volt or Prius, so regardless of whether the EPA gives the Ford F-150 a B or an F, that’s still the type of vehicle he’ll be shopping for” said one source with whom we spoke.

Here is the full list of Congressmen who signed the letter to Jackson and LaHood:

Dale Kildee

Steve LaTourette

Greg Walden

Mary Bono Mack

Andre Carson

Tim Ryan

Bennie Thompson

Cliff Stearns

Joseph Pitts

Dave Camp

Steve Scalise

Bob Latta

Ralph Hall

Tim Murphy

Lamar Smith

Dan Lungren

Dan Burton

Judy Biggert

Geoff Davis

Lee Terry

Mike Rogers

Candice Miller

Charles Gonzalez

Patrick Tiberi

Brett Guthrie

Phil Gingrey

John Sullivan

Scott Garrett

Elton Gallegly

Jim Matheson

Tim Holden

Sam Graves

Mike Ross

Robert Aderholt

Ed Whitfield

G.K. Butterfield

Spencer Bachus

Sue Myrick

F. James Sensenbrenner

Gary Peters

Betty Sutton

John Dingell

Donald Manzullo

John Campbell

Todd Akin

John Barrow

Thaddeus McCotter

Marsha Blackburn

Sander Levin

Mike Simpson

Dave Loebsack

Mark Schauer

Bruce Braley