The long campaign against shoplifting and other ripoffs is finally paying off -- for the merchants, not the perpetrators.

Robert J. Mulligan, chairman of the Metropolitan Washington Board of Trade's retail bureau -- which is gearing up for its 12th annual anticrime campaign -- reports that regional losses from shoplifting fell 2 percent in the last year, the first decline in at least eight years.

In money, that amounts to $10 million, no trivial sum. Still, total shoplifting losses in the 12-month period that ended July 31 were $486 million, a total that also reflects thefts by employes. According to the trade board, the first $100 spent by a family of four in the Washington area goes to pay for the losses.

Bad-check writing also decreased by 3.5 percent, but still totaled $24 million. Bucking the downward trend, credit card fraud rose 4 percent to $5 million.

Who is the most likely shoplifter? According to court records, the board said housewives committed 27 percent of the thievery (and a whopping 55 percent of them in food stores), followed by female teen-agers (14 percent) and male teen-agers (11 percent). Housewives also led by passing 26 percent of the bad checks (65 percent in food stores), with female white-collar workers in second place with 21 percent. But female white-collar workers were tops in credit-card fraud with 24 percent. Adult males ranked low in all categories.