The House decisively rejected all proposed changes in veterans' preference laws yesterday as it moved toward a final vote on president Carter's plan to overhaul the federal civil service system.

The changes, vigorously opposed by organized veterans' groups and rejected last month by the Senate, would have curtailed the advantages in federal hiring and job retention given to most able-bodied veterans and "refocused" them on disabled and Vietnam-era veterans, according to the Carter administration.

Meanwhile, in negotiations over the weekend, supporters of the bill had eliminated the primary obstacle to eventual approval by the House. That obstacle was the opposition of Pro-labor forces, led by Rep. William Clay (D-Mo.). They had been fighting for a change that would have permitted government workers to participate in partisan political activities, an activity barred by the Hatch Act.

Clay was finally mollified by a written assurance from Carter that he would work for approval of a measure to revise the Hatch Act to remove this barrier.

Veterans preference and the issue of whether to tack revision of the Hatch Act onto the civil service bill had been seen as the biggest stumbling blocks to House action. The House bill now conforms more closely to the Carter position, and to the companion bill already approved overwhelmingly by the Senate.

Carter forces knew they had lost the fight for changes in veterans preference yesterday, which, after 1 1/2-hourdebate, the House defeated, 222 to 149, a compromise offered by Rep. David E. Bonior (D-Mich.), chairman of the Vietnam Veterans in Congress, that would have expanded benefits for Vietnam-era combat veterans.

The House then approved, 281 to 88, and amendment offered by Rep. James M. Hanley (D-N.Y.) which has the effect of continuing all current veterans-preference rights.

Hanley characterized the administration attempt to "change the ground rules" on its commitment to veterans in a manner that jeopardized "the integrity of the U.S. government."

Hanley and others who opposed the changes disputed administration contentions that veterans preference denies women and minorities a fair crack at federal job opportunities.

Supporters of the proposed changes, led by Reps. Morris K. Udall (D-Ariz.) Patricia Schroeder (D-Colo.) and Bonior noted, among other things, that male veterans hold 65 percent of government jobs, while women hold only 3 percent.

Bonior asserted that expansion of benefits for Vietnam combat veterans would help reduce the employment problem among younger people.

Chairman Alan K. Campbell of the Civil Service Commission who has led the administration's fight for the changes, said that "we're obviously disappointed" because of the impact on management flexibility and "basic equity."

However, Campbell added, "The management orientation of the legislation overall remains intact."

In the light of the president's written promise of support for separate Hatch Act legislation, Clay said the he was "delighted".

That compromise, prepared by the bill's floor manager, Udall, apparently assured the Carter forces of support of conservative members.

The House committee version of the bill had traveled a much rockier road than one passed by the full Senate last month. For example, the House Committee bill reflected the efforts of prolabor forces.

It would have strengthened employe rights and union power far beyond the point the administration has said it is willing to go and would have enabled federal workers to take an active role in partisan politics, an activity now forbidden by the Hatch Act.

The Senate version, passed 87-1, is closer to the president's position. It gives him virtually everything he asked for with one major exception.

That exception is the modification of veterans' preference rights and federal hiring and job retention. The Senate not only killed the changes the president wanted, but also made some changes in the opposite direction which veteran groups say would strengthen existing advantages given veterans.

The Senate also strengthened the protections provided by the bill for so-called "whistleblowers" - federal workers who expose wrongdoing or waste by their bosses. It also reduced somewhat the power to be given the president's new personnel management arm.

Among the fundamental provisions of the president's plan approved by the Senate were:

Creation of a senior executive service (SES) open on a voluntary basis to some 9,000 federal managers. They would be allowed to trade some of their present job security for higher pay and other rewards for superior performance. They could be dismissed or demoted for poorer work. No more than 10 percent of the SES could be political appointees as opposed to career civil servants.

The House version, under a measure sponsored by Rep. Gladys Noon Spellman (D-Md.), would limit the SES initially to a two-year trial period in only three major departments. It would automatically be expanded governmentwide, at the end of that period if Congress did not intervene.

Pay raises for some 72,500 mid-level managers and supervisors (GS 13-to-15 job rankings) would no longer be automatic, but would be tied to performance.

Managers would have greater flexibility in firings, demotions and other personnel actions through what the administration considers a streamlined appeals process. And for the first time, employes fighting such actions would have the right to choose between statutory civil service appeals route and union arbitration machinery.