third rail

1. N-COUNT The third rail on a railway track is the steel bar which passes electric current to a train. (铁路轨道)第三轨; 输电轨

2. N-SING If you refer to something as the third rail of a political situation, you mean that it would be dangerous to make changes to that thing because people feel very strongly about it. (政治上的)险局[美国英语] ['the' N]

Politicians speak of Social Security as the third rail. To touch it is to commit political suicide. 社保对政客们来说是个高危带电的话题，碰它就等于政治自杀。