Mark Kleiman suggests that Democrats start implying that Republicans want to abolish Medicare. This is a rather common trope among Democrats. But in this case, I don't think it will work.
But in this case, given that Barack Obama actually has proposed substantial changes to Medicare, including a commission in charge of restricting treatments (your belief that we should restrict those treatments is rather beside the point), this doesn't seem like a winning issue for the Democrats. Well informed seniors--the sort who follow these debates--are aware that Medicare is unsustainable, and presumably the reason so many of them oppose health care change is that think that expanding coverage will simply hasten the day of reckoning. Bringing up abolishing Medicare simply reminds them of this point.