third rail
US /ˌθɜːrd ˈreɪl/
UK /ˌθɜːrd ˈreɪl/
1.
a political issue that is so controversial that any politician who dares to broach the topic will suffer a backlash from voters
:
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Healthcare reform has become a third rail issue in American politics.
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No politician wants to touch the third rail of social security.