An effort to declassify American intelligence files on Argentina’s dictatorship failed in Congress on Friday in Washington, disappointing rights activists in Argentina who believe the secret documents could help them identify young people stolen as babies by the military junta. The amendment by Representative Maurice D. Hinchey, Democrat of New York, left, was rejected by a vote of 214 to 194. It would have compelled American intelligence agencies to declassify their files on the 1976-1983 dictatorship, which was closely monitored by United States intelligence agencies. A similar amendment in 1999 resulted in the Chile declassification project under President Bill Clinton, which led to the publication of more than 24,000 documents that helped prosecute crimes against humanity committed during the dictatorship of Gen. Augusto Pinochet. Most of the files on Argentina remain secret.