Recent studies from New York, Los Angeles and, now, the Washington area have shown that cheating immigrants out of their pay is a workplace pandemic. A staggering 58 percent of day workers in the District report having been swindled by employers. It is outrageous that until now, local authorities here and in most of the country have done virtually nothing about it. Indeed, in most places the official response to day laborers who did a job and then got stiffed has been: “Tough luck”.    Deadbeat employers have benefited from the neglect of understaffed state labor departments. Nonpayment of wages has been treated in most places as a civil rather than a criminal matter, subject to light fines. In Maryland Republican Gov. Robert L. Ehrlich Jr. thinks so little of the problem, and of the rights of low-income immigrant workers, that he is abolishing the office within the state Labor Department that would enforce payment-of-wages laws.    Still, as reports of the routine exploitation and abuse of workers reach a critical mass, there are tentative signs of change. In parts of California, New York and Texas, local authorities have been more aggressively confronting scofflaw contractors and other employers who withhold wages. In Virginia a law sponsored by Del. Adam P. Ebbin takes effect this week that will make it a felony for employers to write a bad check to workers for more than $200. Prosecutors in Prince George's County, in an innovative use of an existing statute inspired by the lack of state enforcement, have brought a handful of felony charges against deadbeat employers under a law that forbids theft of services.    Though admirable, the efforts underway to compel employers to do the right thing are more symbolic than muscular. The immigrant day-labor workforce in this region is booming, and so are the rip-offs to which it is subjected. Police and prosecutors' offices have so far paid little attention to a politically voiceless community. That needs to change.