Despite the Obama administration’s best efforts, the lack of affordable housing remains one of America’s most vexing problems. A $1.5 billion transfusion in the 2009 stimulus bill managed to hold the overall homeless population about even over the last four years, and the number of people in two especially vulnerable groups — the chronically homeless and homeless veterans — has actually dropped. But with nearly 3.3 million families with children living in “worst case” situations — spending more than half their incomes on housing or living in hazardous buildings — more must be done to create affordable housing, through rehabilitation or new construction. For starters, that means finally putting money into the National Housing Trust Fund, which was created by Congress in 2008 but was never financed because of the recession. This should be an early order of business for the new Congress. The fund, modeled on successful state programs, would provide subsidies and incentives to preserve, rehabilitate and build housing, primarily for extremely low-income families that earn about 30 percent of the average median incomes in their areas, typically spend more than half their incomes on rent and are disproportionately at risk of slipping into homelessness.