Thousands of recently downgraded federal workers would be spared actual pay and grade cuts under a no-fault demotion bill about to be introduced in the House.

The legislation - outlined here March 22 - is the work of Rep. Robert N. C. Nix (D-Pa.). Nix heads the Post Office-Civil Service Committee that must clear any downgrading protection legislation. He's also had staff members probing recent agency downgrading actions.

The primary thrust of the Nix bill, which may be introducted soon after the Easter recess, is to give guarantees to employees whose jobs are downgraded because of reorganizations or agency audits that their pay and grade will be protected as long as they are in the job. The job, would be "red-lined" as a downgraded position and would be officially downgraded when the incumbent employee leaves.

An important feature of the bill would include a grandfather clause on the pay status of employees subject to demotions within the past two years. Under present law, those affected by no-fault downgradings can keep their salary for two years, but they lose their grade.

The Nix bill would, if details can be worked out, give those recently downgraded employees the same guarantee of grade and pay status as persons downgraded in the future. In other words, an employee whose position had been downgraded within two years (of the time the bill becomes law) would get his or her higher grade back, and keep it and the pay that goes with it. That job, too, would be "red-lined" and earmarked for downgrading whenever the incumbent left.

Although more generous than what the White House had in mind, insiders believe the Nix bill has the best - and maybe only - chance of becoming law this year. There are several other downgrading protection measures in the works. But none of them has the stamp of approval of the chairman of the Committee.

Experts think the White House will go along with the Nix on-fault downgrading protections because it provides the only means President Carter can deliver on his promise that no federal workers would be demoted or lose pay because of reorganization-related shakeups.

The facts of bureaucratic life are that it will be virtually impossible to reshape agencies into the Carter mold without massive downgrading. The "grandfathering" of pay and benefits to reorganization-related downgrades. But the Nix Committee will push to broaden it to include all downgradings that are not the fault of the worker.

Tough new job audits in government are certain to result in many jobs being downgraded, even though the incumbent employee isn't guilty of the alleged inflation that puffed the grade of the job.

Rather than permit the White House to avoid helping only workers whose jobs are cut back for reasons unrelated to reorganization, the House Committee will attempt to see that the protection extends to all no-fault downgradings. It will be an offer the White House - if it wants to minimize bureaucratic foot-dragging on the reorganization - can't refuse.