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An issue or topic that is so controversial that it would immediately damage or destroy one's political career or credibility. An allusion to the electrified rail that powers electric railway systems, its figurative sense is almost exclusively used in relation to politics. Primarily heard in US. I wouldn't even bring it up—trying to withdraw people's social security benefits has long been the third rail of politics. Any talk of dismantling or reforming the current healthcare system has been a political third rail for the last two decades or so.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Something that is dangerous to tamper with, as in Anything concerning veterans is a political third rail. This term alludes to the rail that supplies the high voltage powering an electric train, so called since 1918. On the other hand, grab hold of the third rail means "become energized." Both shifts from the original meaning date from the late 1900s.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.