third rail
US /ˌθɜːrd ˈreɪl/
UK /ˌθɜːrd ˈreɪl/

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.