In the 1930s, liberals (not all of whom were Democrats) passed Social Security over conservative objections. In the 1960s, liberals (not all of whom were Democrats) passed Medicare over conservative objections.
In the 1970s, 1980s, and early 1990s Democrats ran on Social Security and Medicare. They claimed that if conservative Republicans ever took over Congress, those Republicans would slash or dismantle programs that benefited older Americans. Republicans responded by complaining about the attacks; they acted extremely aggrieved, and claimed that this constituted foul play by the Democrats.
Upon winning majorities in Congress in 1994, Republicans promptly attempted to slash spending on Medicare. Upon winning control of Congress and the White House a decade later, Republican President George W. Bush attempted to dismantle Social Security and replace it with a different program. Most conservatives today seem to believe that the failure of Republicans to implement Bush's plan (along with their willingness to follow Bush in expanding Medicare) were terrible sins for which Republicans were justifiably bounced from their Congressional majorities in 2006.
And yet those same conservative Republicans still seem to believe that it is somehow unfair for Democrats to point out that Republicans oppose Social Security and Medicare as they are currently constituted. As Ezra Klein points out today, Paul Ryan manages to complain about being unfairly attacked for ending traditional Medicare in the very same piece that he demands an end to traditional Medicare. As Klein says, "This might be a good reform or it might be a bad reform, but it's undoubtedly a wholesale transformation of Medicare. Ryan should argue that this is a good thing, rather than try to obscure what he's attempting to do."
There is, of course, a larger issue here. Republicans are in fact trying to straddle the rhetoric of the Tea Party crowd -- the rhetoric of small government -- and the reality that most Americans basically like the current level of government programs and services, and Social Security and Medicare recipients love those two programs and don't want a dime cut out of them. Democrats can get in trouble with that too (that's what the symbolic importance of the public option was all about), but it's pretty clearly true that at least for right now, and for a variety of reasons, Republicans are a lot more invested in keeping those who hold unpopular conservative ideas happy than Democrats are in keeping those who hold unpopular liberal ideas happy. Republicans may think it's unfair that Democrats attack them for the positions they take as a result, but it's not exactly clear to me why neutral observers should respect those complaints.