WASHINGTON  While Senators Hillary Rodham Clinton and Barack Obama fight over who has the better health plan for the uninsured, they say little about a more immediate challenge that will confront the next administration, whether Democratic or Republican: how to tame the soaring costs of Medicare and Medicaid.

The two programs, for older Americans and low-income people, cost $627 billion last year and accounted for 23 percent of all federal spending. With no change in existing law, the Congressional Budget Office says, that cost will double in 10 years and the programs will account for more than 30 percent of the budget.

Economists and health policy experts say the federal health programs are unsustainable in their current form, because they are growing much faster than the economy or the revenues used to finance them. The Medicare program is especially endangered; its hospital insurance trust fund is expected to run out of money in 11 years.

But the need for cutbacks is not a popular theme for political candidates wooing voters who want more care at a lower cost.