WASHINGTON — A major U.S. counterdrug aid package for Mexico is under attack by U.S. organized labor, which says Congress should reject the initiative unless tough human-rights conditions are included, according to a letter revealed Friday.

The opposition by the AFL-CIO and other labor groups adds another obstacle to a three-year, $1.4 billion program for Mexico known as the Merida Initiative. It already faces cutbacks for budgetary reasons and objections from human-rights groups, which say that Mexican security forces have a history of abuses.

In an April 30 letter to House Foreign Affairs Committee members Reps. Howard Berman, D-Calif., and William Delahunt, D-Mass., the AFL-CIO said that fighting drug trafficking is "an important and legitimate foreign policy objective."

But the group cited labor abuses in Mexico and considered the strategy "ineffective." Providing eight military helicopters — meant to quickly move security forces to remote places where drug gangs operate — was "of questionable value," the letter said.

House budget writers are expected to cut the first part of the Merida funding from $500 million to less than $300 million.

Merida Initiative supporter Rep. Eliot Engel, D-N.Y., the chairman of the Western Hemisphere subcommittee of the House Foreign Affairs Committee, said at a hearing Thursday that he was "disappointed" at the funding cut and would attempt to overturn the decision.

But with U.S. unions now firmly opposed, passing Merida will become even harder, congressional aides acknowledge. Key senators are also wary.