Medicare's prescription drug program was controversial when it arrived, but a decade later it is widely considered to be a Washington success story. Now, though, the Obama administration is proposing a series of significant changes to fix what critics say isn't broken. Administration officials say they want to help beneficiaries make good choices and save taxpayers money. But some patient groups, pharmaceutical manufacturers and lawmakers in both parties are pushing back against various elements. The regulation would affect several aspects of the Medicare drug program, also known as Part D, ranging from new limits on the number of plans insurers could offer consumers to new rules about what drugs those plans must cover.