For most of the last 40 years - at least since Tip O'Neill or one of his aides coined the term - the third rail of US politics has been Social Security.
It is strange to think about. We are all MMTers now, after all, and what're a few trillion dollars among friends? In 2021, entitlement reform is a third rail in the same way that a high-voltage power line is a third rail. Sure, it will electrocute you, but who is going to go through the trouble of getting to it? We didn't stop talking about entitlement reform because people were too afraid of it. We stopped talking about entitlement reform because neither of the two parties has even the faintest interest in it.
Yet even during its heyday, I would argue that Social Security was never really America's third rail. Politicians talked about it all the time, even if half of their conversations consisted of calling it a third rail. A true third rail would be an issue or policy that everyone was too afraid to even bring up in that context. It would be a topic we were too afraid to discuss at all.