In what he called the "most sweeping reform of the civil service system" since its birth in 1883, President Carter last March sent to Congress a complex plan he hoped would make the vast federal work force more efficient and responsive to presidential direction.

The legislation has now been approved, with alterations, by committees of both House and Senate. Floor action is expected possibly within the next two weeks.

The Senate governmental Affairs Committee's version gives the president most of what he asked for with one major exception: It killed a proposal to curtail the preferences in federal hiring and job retention given to ablebodied veterans, which the administration says is an important element in the president's overhaul plan.

On the House side the Post Office and Civil Service Committee, traditionally more oriented to the interests of federal employes and their unions, made more drastic changes in the bill. It also added some "Christma tree" amendments that some observers believe will doom the bill if they are not removed by the full House.

The major changes made by the House committee include:

Addition of an amendment that would enable federal employes to take an active role in partisan politics, now prohibited by the Hatch Act.

A greatly expanded and strengthened role for federal employes' unions, the number of issues they are allowed to bargain on, the handling of employe grievances, and other matters.

These labor-oriented changes have aroused the ire of many Republicans and of the Chamber of Commerce, who have been supporting the president's original proposals.

The main elements of Carter's civil service overhaul plan include:

A Senior Executive Service (SES): Some 9,000 federal managers, political appointees and career employes alike could volunteer for this program. It would allow them to trade some of their current job security for a chance at higher pay and other rewards for superior performance. They would risk dismissal or demotion for poor performance. As the administration outlines it, the idea is to use a "carrot and stock" incentive system to make managers more responsive to their political bosses, and give the bosses more flexibility in moving managers around.

In the House committee, an amendment by Rep. Gladys, Spellman (D-Md.) was approved, making the SES a 3-year experimental program, limited to just three federal agencies - a limitation that the administration says guts the SES concept.

Incentive Pay: For about 72,500 mid-level managers and supervisors (grades 13 through 15), length of service would no longer guarantee periodic pay raises. Federal employers would be able to "relate pay to performance."

Firing Process: When a manager fires an employe, they would both face a "quicker and clear" appeals process, as the administration outlines its plan.

The red tape and delays of the current system intimidate managers and overprotect employes, according to administration officials.

Employe protections: While giving managers more flexibility, the Carter plan seeks also to spell out the abuses prohibited by law in hiring, firing and other personnel actions.

Under the president's reorganization authority, Carter would split the Civil Service Commission into two bodies, one to represent the interests of managers, under the wing of the White House; and the other to protect employes against the abuses of management. Both those conflicting missions currently are under one roof.

Other portions of the plan would expand the powers of federal employes' unions - though not as far as the House committee version - and curtail veterans preferences (a provision that is retained in the House committee version).