What does it mean when Social Security is called the "third rail" of politics?

Question

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The term "third rail" refers to the electrified track that powers a subway train and is lethal to touch. In politics, Social Security is often referred to as the "third rail" because proposing changes to it, such as cuts or raising the retirement age, can be politically lethal for a politician. Many voters, especially older ones who rely on Social Security benefits, strongly oppose any changes to the program.

What Does It Mean When Social Security Is Called the "Third