Thousands of key government workers - maybe as many as one in 10 - would continue under the legal blanket of the Hatch Act despite a reform measure cleared by the House on Tuesday.

Under strong pressure from the AFL-CIO, the House approved a bill by Rep. William Clay (D-Mo.) to "de-Hatch" most of the 2.8 million career civil servants and postal workers.They've been legally insulated from partisan politics since 1938.

The House-passed bill would allow most to take active, off-duty roles as either campaign managers, regular campaign workers, fund-raisers or even candidates. But many still would be denied entry into the political mainstream.

An amendment to the House bill opens the possibility that many thousands of U.S. workers - estimates range to 280,000 and up - in "sensitive" jobs would remain under the Hatch Act. That would mean they could not take the same active partisan political actions as their "de-Hatched" co-workers.

Nobody - including the backers of the amendment - knows for sure just how many federal workers would remain "Hatched" if the bill, in its present form, clears the Senate. President Carter has said he would sign a Hatch Act reform measure when and if Congress gets it to him.

Government experts, and union leaders who have followed Hatch Act reform closely, estimate that from 10 to 20 per cent of the federal work force could remain under restrictions of the act. Much depends on how the Civil Service Commission would use the discretionary authority granted it to decide who is - and isn't - subject to the Hatch Act.

Most FBI personnel would remain under the Hatch Act, as would CIA employees, workers of the National Security Agency and other "sensitive" intelligence or law-enforcement operations. In addition, thousands of managers or employees involved in contracts, audits or enforcement of civil and criminal statutes also could be kept under the Hatch Act.

Those who might remain subject to the "no-politics" law include large groups of employees in the Federal Communications Commission, Small Business Administration, CAB Securities and Exchange Commission, Federal Trade and Interstate Commerce Commission and other regulatory groups. Many Internal Revenue Service aides would not be permitted to take active roles in politics, and workers involved in inspection (like meat, poultry or health and safety standards) could also be kept under the no politics restriction.

Federal unions, who led the drive for Hatch Act reform, are unhappy about the potentially large group of workers - and members - who could be kept out of the political main stream. They will push for changes on the Senate side. The Senate is expected to be much tougher, and slower, on Hatch Act reform than the House.

The Senate Governmental Operations Committee (successor to the old Post Office Civil Service unit) has six major pieces of legislation to contend with before the Hatch Act will get the spotlight.

Chairman Abraham Ribicoff (D-Conn.) hasn't scheduled Hatch Act hearings and insiders say he will not be stampeded into quick action. Ribicoff voted against Hatch Act reform last year when the Senate and House cleared the measure, only to see it vetoed by President Ford.