Right now we have conservatives simultaneously calling for huge spending cuts and also getting the line’s share of old people’s votes even while the vast majority of non-security spending is on old people. In essence, by first separating the domestic budget into “discretionary” and “entitlement” portions and then dividing the entitlement programs up into “what today’s old people get” versus “what tomorrow’s old people will get” the political class has created a large and vociferously right-wing class of people who are completely immune from the impact of their own calls for fiscal austerity.
Old people vote Republican. Well off people vote Republican. The benefits we give to old people because they are old are the biggest problems with the budget; the biggest problem being health care cost, i.e. Medicare. Well off people pay the lion's share of taxes that provide those benefits.
But what Republicans want to do is to keep lowering the taxes on those well off people who pay for those things while not pushing as hard for lowering the benefits of the well off people who also enjoy the benefits their taxes pay for. And they aren't very concerned about the less well off people who would also have their benefits cut under their proposals, and those are the people who need the benefits the most.
What Yglesias is pointing out is that Republicans can do these things and not suffer the consequences at the polls. Old people will continue to vote for them, on average, even if their benefits get cut. Well off people will continue to vote for them because they like to believe lower taxes is the best thing for them even though everyone's incomes has grown more under Democratic presidents.
Democrats risk making their base angry by passing cuts to entitlements. And contrary to Republicans, they get no credit for cutting the deficit by doing so. Meanwhile, the less well off people who rely on entitlements enjoy less benefits.
So for liberals, not only do we not see the benefits from cutting entitlements at the polls, the people we care about and who need the help suffer. Plus there is the fact that there is plenty of money that could be cut in the defense budget. And Social Security is not a long term budget problem. What talks about cutting SS boils down to is Republicans being ideologically opposed to the idea of SS and being able to use it as a talking point on budget responsibility, which is bullshit because they aren't responsible when it comes to the budget. Just look at the deficits Reagan and Bush left.