The growing popularity of work-for-welfare programs presents a "serious threat" to government employes because states and counties lay off public employes and replace them in some cases with welfare recipients, the president of the nation's largest public sector union said yesterday.

"Workfare has been a disaster for public employes," said Gerald McEntee, president of the American Federation of State, County, and Muncipal Employees (AFSCME), in an interview.

"Thus far, the record has been one of substituting welfare recipients for public employes, and it has been devastating for morale and it encourages layoffs," he said.

McEntee's criticism came after reports that California is about to join several states with experimental programs to require able-bodied recipients of Aid to Families with Dependent Children to work or risk losing benefits.

New York, Pennsylvania, Michigan and Ohio have workfare programs, which AFSCME in some cases has challenged in courts and legislatures.

Social scientists and government officials say these new efforts are a possible model for a national program that could save governments money and help welfare recipients attain work experience that could lead to permanent jobs.

The California program features such "new wrinkles" as expanded job training and child care for welfare recipients, McEntee said, but similar programs have been counterproductive because they result in layoffs and loss of public services. "They are supposed to set up new jobs and provide new services that haven't been done," he said. But, he added, AFSCME has noted cases in other states "where they lay off 75 park workers, and then later they announce that they are going to clean up parks that haven't been cleaned before."

Overtime pay for public employes is also sharply reduced when workfare participants are used to supplement the work force.

In Pennsylvania, which passed a workfare law in 1982, Gov. Richard L. Thornburgh (R) later imposed a state hiring freeze when AFSCME bargaining units had 5,000 unfilled jobs. Workfare recipients were later assigned to perform jobs that had been done by state workers, prompting a union grievance. Last June, a state arbitrator upheld union complaints that the workfare program violated the state's contract, according to AFSCME.

In New York, McEntee said, the union has found several cases in which public employes, laid off for extended periods, collected welfare and eventually were assigned to the same jobs in which they had earned union wages.