A federal "job insurance" bill now in the works would protect the pay, grade and promotion potential of the hundreds of thousands of government employee who face downgradings because of agency actions or reorganizations.

It's been estimated that three of every 10 civil servants are candidates for job downgrading over the next few years either because of tough new desk audits or White House shakeup and consolidation plans.

The proposal being drafted for Chairman Robert N. C. Nix (D-Pa.) would "grandfather-in" the grade, pay and promotion chances of federal workers whose jobs are downgraded through no fault of their own, they would be guaranteed their present grade, and pay if the position they now occupy is reduced in grade or status. The job would not officially be downgraded until the incumbent left it.

The bill, which will be handled by Nix's Post Office-Civil Service Committee doesn't have White House approval - yet, but it will be introduced soon. Carter aides are expected to endorse it because it would blunt internal bureaucratic opposition to the reorganization and also make it possible for the President to deliver on his promise to civil servants that none of them will be hurt because of his reorganization of the government.

Under present law federal workers, subject to no-fault downgrades are demoted but can be kept on their present salary for two years. While the so-called saved pay law is a great help to the worker hit by downgrading it does nothing to prevent the demotion, requiring the employee who took a job at a specific grade on good faith to start all over again, at a lower rung in the career ladder, seeking promotions and better jobs.

The Nix bill would allow jobs to be downgraded but would not affect the incumbent. He or she would remain at current grade, and be eligible for promotions just as if the downgrading had not taken place.

Only when an employee left a job that had been downgraded would the job actually be downgraded. The individual coming into that job would then take it at the lower - and presumably correct - grade.

Federal officials who have been consulted about the proposed job insurance bill believe it would cost the government relatively little over the short-haul. Many believe that in the long run it could save money by permitting reorganizations and downgradings to be made with the downgrading results delayed until the incumbents leave their jobs.

The plan can't come to soon for many government workers. Congress, the White House and the General Accounting Office have concluded - in different studies - that as many as 30 per cent to 40 per cent of the jobs in some career fields are overgraded. Both Ford and Carter have made strong stamements about the evils of "grade creep" in government, and its cost to the taxpayers.

As a result of the focus on overgrading (estimates are that from 5 per cent to 10 per cent of some jobs studied are undergraded) agencies are auditing positions and downgrading has taken place here and in the field.