Medicare covers people over 65 and some with disabilities. Medicaid covers low-income people and is jointly funded by states and the federal government. Together they cover a large share of Americans and represent a large and growing share of the federal budget.
Medicare's financing is split across trust funds and general revenue. The hospital insurance trust fund is supported by payroll taxes and faces projected shortfalls as the ratio of workers to beneficiaries falls.
Medicaid is administered by states within federal rules, so eligibility and benefits vary considerably. It is also the largest payer for long-term care, including most nursing home residents, a function often overlooked in debate.
Demographics drive the long-run picture. An aging population means more beneficiaries supported by proportionally fewer workers, independent of any policy choice.