This year nearly 55 million Americans will receive $727 billion in Social Security benefits, which are funded primarily through dedicated payroll taxes. While the average Social Security benefit — about $1,200 a month — is relatively modest, roughly half of the nation's retirees would be below the poverty line without it.

See also: Meet the 2012 GOP presidential candidates.

Little wonder that Social Security is often called the "third rail" of American politics: Like a railway's power line, the issue can be dangerous to touch. But with the surpluses in the program's trust funds projected to bottom out by 2036, Social Security's future is again the subject of spirited debate as the 2012 Republican primaries approach.

AARP invited the six leading GOP candidates to address Social Security and three other subjects — Medicare, retirement security, and jobs and the economy. Four of the candidates (Michelle Bachmann, Newt Gingrich, Ron Paul and Rick Perry) accepted AARP's invitation; two (Herman Cain and Mitt Romney) declined.

Watch the Social Security segments of the 20-minute candidate interviews in the video player above. Dave Price of WHO-TV, the NBC affiliate in Des Moines, Iowa, interviewed the candidates on Nov. 4.