Last year more than 47 million Americans — more than 83 percent of them aged 65 and older and the rest disabled — received a total of $516 billion in health insurance benefits under the Medicare program, which is financed by payroll taxes. Before Medicare was enacted in 1965, only 51 percent of people 65 and older had health care coverage; today, they're the only age group with virtually universal coverage.

See also: Meet the 2012 GOP presidential candidates.

Initially decried by its opponents (chief among them the American Medical Association) as socialized medicine, Medicare has become one of the government's most popular programs. But the program faces serious fiscal challenges.

Today 3.9 employees pay taxes into Medicare for every older American who's receiving benefits; that ratio is projected to drop to 2.4 by 2030 as increasing numbers of boomers retire. Fast-rising health care costs threaten to push the program even more out of actuarial balance.

With its hospital-insurance trust fund now projected to bottom out in 2024, Medicare is again at the center of the debate over the government's long-term fiscal health — and a key political issue as the 2012 Republican primaries approach.

AARP invited the six leading GOP candidates to address Medicare and three other subjects — Social Security, 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 Medicare 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.