The American Medical Association, which last month achieved a stunning victory in the House, discovered Tuesday that it has far less influence in the Senate.

As the Senate barreled toward a likely showdown vote Thursday on legislation to overhaul the nation’s civil litigation system, it firmly rejected an amendment, long sought by the AMA. The amendment would have limited the compensation patients can receive for “pain and suffering” in medical malpractice suits.

Doctors have long complained that steep awards for pain and suffering have driven up malpractice insurance rates to ruinous levels and eroded the doctor-patient relationship. But they could not get Congress to even consider their complaints.

Last month, however, with little debate, the House established a cap of $250,000 on awards for pain and suffering in all courts, both state and federal. The Senate showed far less sympathy Tuesday as it defeated by a vote of 56 to 44 an amendment that would have capped pain and suffering awards at $500,000, twice the ceiling enacted by the House.

The vote came after several senators recounted medical horror stories, tales of victims of medical incompetence and mistakes.

Dr. James S. Todd, the executive vice president of the medical association, said the Senate vote was disappointing, but added that the association was encouraged by another amendment sponsored by Sen. Mitch R. McConnell, R-Ky., which passed Tuesday.

That amendment would set a cap on punitive damages in medical malpractice cases.