A decades-old tax credit designed to spur cures for rare diseases has been so successful that it’s now become a target in the House Republican tax plan.

The proposal under consideration would end the tax breaks for development of what are called orphan drugs. Ending the credit used by big and small drug companies could save the government an estimated $54 billion over the next decade, an effort to help offset some of the anticipated losses in revenue if other Republican tax cut provisions become law.

With the Senate poised to offer its own broad tax plan soon, many details are still in flux. But if the orphan drug tax credit is eliminated, the move would represent a rare defeat for the powerful pharmaceutical industry.

The tax credit is part of the popular 1983 Orphan Drug Act, and is one of a host of incentives that supporters say have led to the approval of more than 500 new and much-needed drugs for those rare diseases that each affect fewer than 200,000 people. But the program has also come under scrutiny because critics say that some major drugmakers have exploited it by obtaining the orphan designation for billion-dollar blockbuster drugs like Humira for treating rheumatoid arthritis and Crestor, the cholesterol drug, that were already on the market.