A new downgrading threat is coming up next month for thousands of U.S. workers despite White House promises and congressional attempts to write a no-fault demotion policy covering all civil servants.

The new push will begin in June. That is when the Civil Service Commission will end authority of more than 70 agencies to defer demotions of workers found to be in overgraded jobs. According to a new study by the CSC, an estimated 150,000 government employes are now overgraded and overpaid although only a handful of those jobs actually have been identified and earmarked for downgradings.

CSC ordered the demotion freeze several months back to give agencies time to apply for special permission to put the demotions off for at least another year.

In June the Civil Service Commission, will order most agencies to carry on with their personnel housekeeping and grade-cutting operations.

That means that demotions must be made within four biweekly pay periods following June 4 for jobs that already have been identified and certified as overgraded. The only thing that can save workers from demotions would be for Congress to move quickly on the Nix bill. As reported here Saturday, the House Post Office-Civil Service Committee plans to clear the "no fault" demotion protection language by the third week in May. If it appears that the bill by Chairman Robert N.C. Nix (D-Pa.) is really going somewhere, CSC might again allow agencies to delay downgradings.

In the meantime, agencies still have the opportunity to plead for special consideration that will allow them to defer downgradings through December 1979 if they can convince CSC they are getting grade levels in order.

The proceed-with-demotions order will not affect nearly a dozen agencies that already have been given permission to delay no-fault demotions (except in reduction in force situations) until December 1979. Ironically, those agencies make up early 65 percent of the federal workforce, meaning they do not have to downgrade but others - unless they get the dispensation - do.

Agencies that have been given authority to defer downgradings until late 1979 are:

Defense Department, including Army, Navy, Air Force and Marine Corps . . . Veterans, Administration . . . HEW . . . Internal Revenue Service International Communications Agency (formerly USIA) . . . General Services Administration, and Agriculture.

In addition, Agency for International Development headquarters and National Aeronautics and Space Administration headquarters have demotion deferral authority. Customs Service has it for secretaries (GS 318 series) only.

CSC now is studying requests for the demotion deferral authority from the Bureau of the Mint and from Housing and Urban Development, which is undergoing yet-another re-organization.

Agencies and departments not listed here will be affected by the demotion deferral freeze that will end in June, unless Congress says employes in the meantime or agencies request and get authority to continue the freeze.

Organization of American States is expected to begin making major changes in its staff salary system later this year. The international organization, funded primarily by the U.S. taxpayers, has hired a management consultant firm, Hay Associates, to compare OAS jobs, pay and benefits with other international groups, private industry and the federal government.

If the OAS makes changes it probably will inspire other international groups, who employ thousands of Americans and foreign nationals here, to take another look at their pay and grading procedures.