Ending birthright citizenship for the children of illegal immigrants and raising the retirement age for Social Security are two causes Republicans tend to favor more than Democrats. But different groups of Republicans are enthusiastic about each idea: very roughly speaking, the base and the establishment, respectively. (There is, of course, some overlap: Ted Cruz, Lindsey Graham, and Rand Paul are for both ideas.)
These ideas seem to me to have a few things in common. They’re neither necessary nor sufficient to solve the underlying problems with which they are associated (too much illegal immigration and insolvent entitlements); campaigning for them would carry very significant political costs; and they have nonetheless becomes tests of seriousness about the underlying problems.
Raising the retirement age doesn’t have to be done to make Social Security solvent: The growth of benefits for people with high lifetime earnings could instead be moderated. That will have to be done anyway, even if the retirement age is raised. (Obviously, I’m not considering raising it to 100. Then, it’s true, you’d have a nice surplus.) And trying to raise the retirement age opens you up to a potent line of attack–what about the guy who’s had to do backbreaking labor all his life?–that changing the benefits formula wouldn’t. Notice that Paul Krugman, in attacking Republicans this week for being willing to make Social Security solvent, merely mentions the idea of changing the formula and concentrates his fire on the retirement-age proposal.