While the House quickly passed legislation Friday to provide an additional $2 billion to the nearly depleted “cash for clunkers— program, its fate in the Senate — where a possible filibuster to stop the bill looms — appears much murkier.Senate Majority Leader Harry Reid (D-Nev.) hopes to bring up the clunkers measure before leaving town Aug. 7, spokeswoman Regan Lachapelle said.But Sen. John McCain (R-Ariz.), who opposed the original $1 billion for the program included in last month’s war supplemental package, told Fox News that he would filibuster the House-backed measure if it comes up next week. A spokesman for Sen. Tom Coburn (R-Okla.) said his boss would also support a filibuster.Further complicating matters are Sens. Dianne Feinstein (D-Calif.) and Susan Collins (R-Maine), who have concerns about how effective the program will be at reducing oil consumption and increasing auto sales.In a letter sent Friday to Transportation Secretary Ray LaHood, the two reiterated a request initially made on July 10 for information on the program.“Without strong data on the effectiveness of the program, it will be more difficult for Congress to evaluate and improve the current program,— the letter stated.Collins and Feinstein sponsored a separate car measure last month that included tougher fuel efficiency standards for new car purchases under the program, and they were promised any extension of the current program would reflect their legislation.