For most of its existence, the main shortcoming of the Section 8 program, created in 1974 as an alternative to ghettoizing public housing projects, was its inability to keep up with demand. But the recent economic boom in Philadelphia, long one of the most affordable big cities in the Washington-to-Boston corridor, has led to rent increases even in poor and working-class neighborhoods, and many landlords are now refusing to accept the vouchers when they can get higher rents, without the bureaucratic red tape that plagues the program, on the open market.

A survey by the nonpartisan Urban Institute, commissioned by the Department of Housing and Urban Development and released in August, documented the problem in stark terms. It found that 67 percent of Philadelphia’s landlords refused to even consider voucher holders, some candidly citing the low subsidies and their desire to cash in on a hot market. The rejection rates were even higher in Fort Worth and Los Angeles, where three-quarters of landlords turned away Section 8 tenants.

Put at risk by these market forces is the future of a core federal housing program that now serves 2.2 million low-income families and was started with a simple goal: to enable those families to escape neighborhoods increasingly segregated along racial and economic lines for a place with decent housing and better schools, stores and transportation.

“It is a crisis,” said Rasheedah Phillips, managing attorney at Community Legal Services of Philadelphia’s housing unit, which defends tenants in court. “It used to be that Section 8 was basically a guarantee of shelter for families, for the elderly, for disabled people, but now it’s becoming much harder for tenants to get landlords to take the vouchers. And it’s only getting worse as the market heats up.”