The bishops said in a statement Tuesday to Health and Human Services that the prospective new rules don't do enough to protect religious liberty.
The mandate requires employers to provide health insurance that includes birth control for workers. The plan sparked protests from faith leaders because it included most religious nonprofits such as hospitals and colleges.
President Barack Obama offered a compromise. He said insurers would bear the cost of the birth control instead of religious employers. However, the bishops argue that the compromise still makes their insurance plans a conduit for contraceptives.
The administration is reviewing public comments before making any final decision.