third rail

US /ˌθɜːrd ˈreɪl/
UK /ˌθɜːrd ˈreɪl/
"third rail" picture
1.

tercer riel, tema tabú

a political issue that is so controversial that any politician who dares to broach the topic will suffer a backlash from voters

:
Healthcare reform has become a third rail issue in American politics.
La reforma de la atención médica se ha convertido en un tema de tercer riel en la política estadounidense.
No politician wants to touch the third rail of social security.
Ningún político quiere tocar el tercer riel de la seguridad social.