“While we are very happy with the result, we won’t be shouting it from the rooftops because we aren’t out of the woods yet,” Mr. Blumenauer’s office said in an e-mail in early November to people working with him on the issue. “This regulation could be modified or reversed, especially if Republican leaders try to use this small provision to perpetuate the ‘death panel’ myth.”

Moreover, the e-mail said: “We would ask that you not broadcast this accomplishment out to any of your lists, even if they are ‘supporters’  e-mails can too easily be forwarded.”

The e-mail continued: “Thus far, it seems that no press or blogs have discovered it, but we will be keeping a close watch and may be calling on you if we need a rapid, targeted response. The longer this goes unnoticed, the better our chances of keeping it.”

In the interview, Mr. Blumenauer said, “Lies can go viral if people use them for political purposes.”

The proposal for Medicare coverage of advance care planning was omitted from the final health care bill because of the uproar over unsubstantiated claims that it would encourage euthanasia.

Sarah Palin, the 2008 Republican vice-presidential candidate, and Representative John A. Boehner of Ohio, the House Republican leader, led the criticism in the summer of 2009. Ms. Palin said “Obama’s death panel” would decide who was worthy of health care. Mr. Boehner, who is in line to become speaker, said, “This provision may start us down a treacherous path toward government-encouraged euthanasia.” Forced onto the defensive, Mr. Obama said that nothing in the bill would “pull the plug on grandma.”

A recent poll by the Kaiser Family Foundation suggests that the idea of death panels persists. In the September poll, 30 percent of Americans 65 and older said the new health care law allowed a government panel to make decisions about end-of-life care for people on Medicare. The law has no such provision.