Under current law, the Defense Department provides such treatment, including in vitro fertilization, to injured service members through a special insurance program. Lawmakers on both sides of the aisle have long sought to extend those benefits to veterans — hundreds of whom have suffered injuries to their reproductive organs in Iraq and Afghanistan — who receive care through the Department of Veterans Affairs.

In April, House and Senate committees passed appropriations bills that would allocate money for assisted reproductive technology at the Department of Veterans Affairs. Though both measures stalled over unrelated partisan spending fights, Congress is likely to take up the issue again in the fall, when spending provisions are likely to dominate the floors of both chambers.

Critics of Mr. Harris’s amendment, which two Republicans joined Democrats in trying to scuttle, say it could threaten not only the treatment proposal for veterans but also the existing program for service members who receive reproductive services at six military treatment facilities. As of last month, 24 seriously ill or injured active-duty service members had used those services.

The critics say the amendment is problematic because patients undergoing in vitro treatments often create more embryos than they may need or use. Frozen eggs are sometimes thawed and implanted if the first treatment attempt fails. But other eggs, viable or not, do not always survive the treatment process, and can be destroyed.

If Mr. Harris’s prohibition takes effect, patients would probably be forced to implant all the embryos at once, which can be dangerous, or lose the ability to control what happens to any unused embryos.