Social Security is the single largest federal program.
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Sen. Rubio gave a speech at the National Press Club addressing some of America’s retirement policies and the problems we’re facing in a future where a much larger percentage of the population are retirees.

Charles Blahaus, a trustee for Social Security and Medicare, wrote about this at Economics21:

Perhaps the most significant policy element in the Rubio speech was his embrace of specific reforms to stabilize Social Security’s finances. Social Security is the single largest federal program; it also faces the greatest financing shortfall of all federal trust funds including Medicare’s. Reforms to maintain Social Security solvency would improve the federal fiscal outlook dramatically as well as strengthen the foundation of most Americans’ retirement security.