Third rail of politics

The third rail of a nation's politics is a metaphor for any issue so controversial that it is "charged" and "untouchable" to the extent that any politician or public official who dares to broach the subject will invariably suffer politically.

It is most commonly used in North America. Though commonly attributed to Tip O'Neill,Speaker of the United States House of Representatives during the Reagan presidency, it seems to have been coined by O'Neill aide Kirk O'Donnell in 1982 in reference to Social Security.

The metaphor comes from the high-voltage third rail in some electric railway systems. Stepping on this usually results in electrocution, and the use of the term in politics relates to the risk of "political death" that a politician would face by tackling certain issues.

A wide range of issues might be claimed detrimental to politicians tackling them, but those below have all been explicitly described using the "third rail" metaphor:

Jim Leech and Jacquie McNish: The Third Rail: Confronting our Pension Failures, Signal Books, 2013. http://www.amazon.ca/The-Third-Rail-Confronting-Failures/dp/0771046634

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