As the safety net has expanded over the past generation, the food stamp rolls have doubled, Medicaid enrollment has tripled and payments from the earned-income tax credit have nearly quadrupled.

But one major form of aid has grown more scarce.

After decades of rising rents, housing assistance for the poorest tenants has fallen to the lowest level in nearly a quarter-century. The three main federal programs for the neediest renters — public housing, Section 8, and Housing Choice Vouchers — serve 287,000 fewer households than they did at their peak in 2004, a new analysis shows. That is a 6 percent drop, while the number of eligible households without aid grew by about a quarter, to 15 million.

“We’re not just treading water — we’re falling further behind,” said Chris Herbert, the managing director of the Harvard Joint Center for Housing Studies, which prepared the analysis at the request of The New York Times. “That was an eye-opener, even for me.”

In an exception to the trend of falling aid, the Low-Income Housing Tax Credit helped build several million subsidized apartments, but most are not affordable to the neediest renters without additional aid.

Nearly two-thirds of renters in the bottom income quintile face “severe cost burdens,” the Harvard analysis found, meaning they spend more than half their income for shelter. That is a record high, up from about half two decades ago, and it coincides with government findings of record homelessness this year.

Unlike entitlement programs such as food stamps or Medicaid, which automatically grow with need, rental aid is set by Congress each year and reaches only a small share of eligible households.