Two senators wrote Transportion Secretary Ray LaHood asking for an evaluation of the the "cash for clunkers" program before the Senate takes up a vote to extend the program's funding this week.

Sens. Dianne FeinsteinDianne FeinsteinA guide to the committees: Senate Dem: Trump's China trademark looks like a quid pro quo Senate advances Trump's Commerce pick MORE (D-Calif.) and Susan CollinsSusan CollinsLeaked ObamaCare bill would defund Planned Parenthood GOP lawmaker at town hall calls on Trump to release his tax returns GOP senator won't vote to defund Planned Parenthood MORE (R-Maine), two principal authors of the initial bill, asked LaHood for key statistics on the number of cars purchased under the program, and the extent of the environmental impact expected to be gained by the program.

"We believe that Congress needs this information immediately in order to evaluate the program's effectiveness in attaining its two goals of stimulating automobile sales and reducing oil consumption," the senators wrote. "Without strong data on the effectiveness of the program, it will be more difficult for Congress to evaluate and improve the current program."

The extension of the program may face a bit more of an uphill climb in the Senate compared to the House, which easily passed the fast-tracked bill. Sen. John McCainJohn McCainTrump budget to boost defense spending by B Father of slain Navy SEAL wants investigation A stronger NATO for a safer world MORE (R-Ariz.), for instance, pledged to filibuster an extension.

(Hat tip: SenateUS)