Federal employe unions told a House subcommittee yesterday that they absolutely oppose compulsory inclusion of new federal workers in Social Security, but Rep. Bill Archer (R-Tex.) said the Treasury pays about four-fifths of the costs of the Civil Service retirement system and "The American taxpayer is just not going to be willing to bear that kind of burden."

Archer, clashing sharply with officials of the National Association of Letter Carriers, American Postal Workers Union and several other federal employe groups, challenged their assertions that putting new federal employes under Social Security would both bankrupt the Civil Service retirement system and increase government costs.

Earlier, spokesmen for the American Council of Life Insurance, National Association of Life Underwriters, American Jewish Committee and American Jewish Congress endorsed the Social Security rescue plan recommended by a special presidential advisory commission, although the two Jewish groups expressed concern about future benefit restrictions in the package.

Archer, with strong support from Sylvester Schieber, an actuary and research director of the nonprofit Employe Benefit Research Institute, said that to keep the Civil Service retirement fund solvent the Treasury now has to contribute about $4.50 for each $1 contributed by workers.

Consequently, Archer said, "Every new employe you have in the fund is a greater drain on the fund than a help to the fund."

Schieber told the subcommittee that, contrary to assertions by many federal employe groups, the total cost of paying off benefits to those already working for the government (who would not be affected by the Social Security plan) plus putting new federal workers under Social Security and giving them supplementary benefits would be no greater than the likely cost of the current arrangement.

And he said many workers, particularly in low-income and minority groups, would get far better disability aid and family protection.