Something strange is happening in the Republican primary — something strange, that is, besides the Trump phenomenon. For some reason, just about all the leading candidates other than The Donald have taken a deeply unpopular position, a known political loser, on a major domestic policy issue. And it’s interesting to ask why.

The issue in question is the future of Social Security, which turned 80 last week. The retirement program is, of course, both extremely popular and a long-term target of conservatives, who want to kill it precisely because its popularity helps legitimize government action in general. As the right-wing activist Stephen Moore (now chief economist of the Heritage Foundation) once declared, Social Security is “the soft underbelly of the welfare state”; “jab your spear through that” and you can undermine the whole thing.

But that was a decade ago, during former President George W. Bush’s attempt to privatize the program — and what Mr. Bush learned was that the underbelly wasn’t that soft after all. Despite the political momentum coming from the G.O.P.’s victory in the 2004 election, despite support from much of the media establishment, the assault on Social Security quickly crashed and burned. Voters, it turns out, like Social Security as it is, and don’t want it cut.

It’s remarkable, then, that most of the Republicans who would be president seem to be lining up for another round of punishment. In particular, they’ve been declaring that the retirement age — which has already been pushed up from 65 to 66, and is scheduled to rise to 67 — should go up even further.