(FORTUNE Magazine) – -- Nexis, normally unflappable, seemed instantly daunted by the request we put to it the other day. We had asked how many stories in our favorite database featured the phrase ''affordable housing.'' After an uncharacteristically long pause, Nexis noted that the total would be over 500 and asked incredulously if we wanted them all listed. We hit the Yes button, promising to read every one. There followed another hiatus while the hard-disk light kept winking, and finally the answer was vouchsafed: 9,853 stories. In candor, we read only 80 of them and yet feel that we definitely have the idea. Like rent control in New York (see the item above), ''affordable housing'' is more popular with politicians than economists. The phrase, which at first blush sounds merely descriptive, like ''suede shoes,'' is now widely understood to be Esperanto for a huge new avalanche of taxpayer-supported low- income housing construction that will be sponsored by the federal government. Helping to promote the avalanche, a.k.a. the National Affordable Housing Act now motoring through Congress, are three low-grade ideas. -- The idea that the existence of homelessness proves the existence of a housing shortage. The idea would come close to being a sequitur if the 450,000-odd homeless in America were not so heavily burdened by drugs and mental illness (more than one-third of all cases, according to Richard Freeman of Harvard, a meticulous scholar whose estimate of the homeless total we are also adopting). The idea would also look more plausible if the national vacancy rate were at least on the low side; in fact, the rate for rental housing was recently up to 7.3%, not far from the postwar high. The aggregate data simply do not support the idea of a connection between homelessness and the housing stock. The connection is nevertheless guaranteed to be a major theme of the forthcoming march on Washington (it is now scheduled for October 7), which will agitate for more federal action on affordable housing. -- The idea that federally subsidized housing is the obvious solution to the affordability problem. Somewhat astonishing is the durability of this idea in the wake of the scandals at the Department of Housing and Urban Development. The sound you cannot hear these days is a Congressman proposing to expand Reagan's modest program of housing vouchers for the poor. (The deal is that they can rent absolutely anything they want, with the aid of certificates spendable only on housing.) -- The idea that every last community needs to have its own supply of new low- income housing. Some states have pushed this idea to quite incredible lengths, although only in New Jersey, so far as we know, have regulators called for the demolition of decent middle-class housing when that is what it takes to create space for more meritorious ''affordable'' stuff. You can see why we are resisting the other 9,773 stories.