Digby kicked off a great discussion over the weekend, picked up by Krugman, here, when asking why so much attention on Social Security and not the much more fiscally problematic Medicare.
When medical expenses are big, they’re big; even the very affluent are grateful when Medicare pays the bills for their mother-in-laws bypass or dialysis. The importance of Medicare, in short, is obvious to all but the very rich.
Social Security, by contrast, is something that matters enormously to the bottom half of the income distribution, but no so much to people in the 250K-plus club. A 30 percent cut in benefits would represent disaster for tens of millions of Americans, but a barely noticeable inconvenience for VSPs (Very Serious Persons) and everyone they know. A rise in the retirement age would be a vast hardship for people who do manual labor, but if anything a gift to VSPs, who don’t want to step aside in any case. And so on down the line.
So going after Social Security is a way to seem tough and serious — but entirely at the expense of people you don’t know.
To emphasize that point, Yglesias added this chart:

Even though Social Security is only a very mildly redistributive program, inequality of wealth is such that it’s a vital element of the bottom 60 percent’s living standards but kind of small beer to the top twenty percent. But I would say the other thing here on the Medicare / Social Security contrast is that Medicare isn’t just a subsidy program for old people. It’s also a subsidy program for doctors, nurses, hospital administrators, pharmaceutical executives, etc. Those people have lobbyists, many of their professions are well-respected, and many members of the political/media elite have siblings, cousins, college buddies, and even spouses who work in those fields.
It's politically easier to insist that we all have to "share the burden" when the share of that burden is disproportionately shouldered by the people with the least power. The only power most seniors and disabled people have is at the ballot box, and that's where Dems are putting themselves in the most danger by flirting with the third rail.