WHEN it comes to federal action to relieve the homelessness endured by up to 3 million Americans, promises have been plentiful. But there's a hint, at least, that expressions of concern could become concrete programs. Congress is starting to wrestle with an underlying cause of persistent homelessness - the shrinking supply of low-cost housing. This problem defies emergency relief efforts, such as those included in the McKinney homelessness act of 1987.
A big investment is unavoidable, and at least a significant share of that investment will have to come from Washington. The private sector must be involved, certainly, but needed incentives will come from federal grants and tax breaks.
The legislation just introduced by Sens. Alan Cranston (D) of California and Alfonse D'Amato (R) of New York includes a $4.1 billion program to provide grants to local and state governments and private housing agencies to create more housing.
The price will stop some, but the concept needs sympathetic consideration - as do parts of this bill and others that deal with rental credits, easier financing for home-ownership, and other housing remedies.
If Americans want further evidence of the need for strong housing legislation - other than the homeless they pass on their streets - the imminent conversion of 300,000 low-cost housing units to market rents should suffice.
The builders of these federally subsidized units were given the option of prepaying their mortgages after 20 years and thus acquiring the freedom to change clientele. Soaring real estate markets have given owners a huge incentive to do just this.
Local governments and tenant groups are rushing to prevent a massive loss of housing for low and middle-income people, but a significant number of units are likely to be lost nonetheless.
The administration is fully aware of the housing need, and its point man on this issue, indomitable Housing and Urban Development secretary Jack Kemp, is busily trying to figure out how to aid the homeless and stimulate economic development in the areas that most need low-cost housing - all within the dimensions of a pinched budget.
Mr. Kemp has made a good start by actually meeting with the homeless and appointing a new, and likely much more effective, chairman to the Interagency Council on the Homeless. That agency, charged with dispersing information on how to utilize benefits under the McKinney act, fell moribund during Ronald Reagan's watch.
Kemp and others in Washington determined to shoulder the housing issue need every encouragement in their efforts to show that the acronym HUD does not stand for Homelessness and Urban Decay.