third rail

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

troisième rail, sujet sensible

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 réforme des soins de santé est devenue un sujet sensible en politique américaine.
No politician wants to touch the third rail of social security.
Aucun politicien ne veut toucher au troisième rail de la sécurité sociale.