Third rail

Etymology

The metaphor comes from electric railways, where the “third rail” carries the high-voltage current — touch it and you die. The phrase entered American politics around 1980, famously applied to Social Security: “the third rail of American politics — touch it and you’re dead.”

noun phrase · /thurd rayl/
A political issue so charged that touching it is likely to destroy a career.
What it names
The topics politicians avoid because any position taken provokes fierce, disqualifying backlash.
Example
For decades, proposing cuts to popular benefits was the third rail no candidate would touch.
Related idea
The third rail is a danger to whoever raises it; a wedge issue is raised deliberately to split an opponent’s coalition, and the Overton window describes the broader range of positions currently safe to hold at all.
noun · /thurd rayl/
An electrified rail running beside the track that supplies power to a train — deadly to touch.
What it names
the live power rail of an electric railway
Example
Signs along the platform warn that the third rail carries hundreds of volts.
Related idea
The political metaphor keeps the literal lesson: certain issues in a culture war are the rail you do not touch and survive.