The Carter administration's move to curtail the solidly entrenched preference for military veterans in federal hiring emerged yesterday as the most controversial issue in the newly announced plan to overhaul the civil service system.

"My feeling is that the average person running for reelection isn't going to want to take on the veterans of this country -- and consequently, that is one segment of the bill that is in real trouble," Sen. Abraham A. Ribicoff (D-Conn.), chairman of the Governmental Affairs Committee, said in a television interview. Ribicoff's committee will consider the administration plan.

The controversy surrounding Carter's proposal to cut back on preferential hiring of veterans -- a move opposed by the American Legion and Veterans of Foreign Wars -- underscored what appeared to be considerable uncertainties confronting the administration's civil service plans, as the center of debate about the package shifted to Congress.

While some key members of Congress expressed support for Carter's main proposals, many also expressed doubt that substantial parts of the administration's plan could win congressional approval this year -- an election year in which Congress is expected to recess early for the fall political campaigns.

The Carter administration's proposals also set off a sharp clash among three of the major labor unions that represent federal government workers and drew mixed reactions from other lobbying groups.

The American Federation of Government Employees, an AFL-CIO affiliate and the largest of the federal employe unions, hailed the administration's plan as "a major breakthrough" that would lead to broader collective bargaining rights for government workers. AFGE's support stemmed from Carter's proposal for establishing an independent agency to oversee labor-management relations. That agency would be known as the Federal Labor Relations Authority.

"If others want to cast stone at that accomplishment, let them," 'AFGE president Kenneth T. Blaylock said in a statement yesterday.

AFGE's support for Carter's plan already had come under attack from two other federal employe unions --Employees, the second largest, and the National Federation of Federal the National Treasury Employees Union. The treasury workers group, the third largest federal employe union, started a campaign last year to expand its membership to include employes outside Treasury agencies.

The presidents of both NFFE and the Treasury union denounced the Carter proposals, warning that they may presage a return of the "spoils" system -- a form of political patronage that the civil service system was designed to eliminate when it was started in 1883.

Carter's proposed civil service overhaul gained only lukewarm support from consumer advocate Ralph Nader, who termed the plan a "halfway reform" that failed to provide a mechanism through which citizens could seek action against alleged abuses by government workers. "Citizens need an ombudsman to investigate charges of official illegality," Nader said in a statement.

Common Cause, another public advocacy group, showed greater enthusiasm for the Carter package, calling the administration's proposal's "excellent as an opening gun in a hard battle."

Despite the wide-ranging debate and uncertainties that engulfed the administration's proposals, Carter and key administration officials expressed cautious optimism that Congress would move quickly to approve the plans.

"We obviously are not certain that it will go through this year," said Civil Service Commission Chairman Alan K. Campbell, a key administration proponent of the civil service overhaul, in an interview yesterday afternoon. But he described himself as optimistic and predicted that the plans had a "better than even chance" of rapid congressional enactment.

Methods of selecting, promoting and firing government employes have been mired in controversy throughout much of American history. The spoils system, often viewed as the creation largely of President Andrew Jackson, came to an end in 1883 after years of reform campaigns, spurred by the assassination in 1881 of President James A. Garfield. His assassin has been described as a disappointed office seeker who had been refused appointment at U.S. consul in Paris.

The system of giving military veterans preferential treatment when they apply for federal government jobs dates primarily from legislation enacted in 1944, although presidents, including George Washington and Abraham Lincoln, made efforts to assist war veterans.

In recent times, proposals to revamp the federal merit system have proliferated, although few have led to substantial changes. According to Civil Service Commission officials. 20 major proposals have been made by government commissions and private groups for revising the federal government's personnel management system since 1937. Among them were recommendations by a commission headed by former president Herbert hoover in 1949 and 1955.

Against this background, CSC Chairman Campbell and other administration aides have conducted since last June an intensive drive to gain support for the Carter administration proposals announced in detail yesterday. According to a CSC spokesman, Campbell and other officials visited 14 cities outside Washington during this campaign.

In one of the most dramatic episodes during this drive, Campbell chose to disclose the proposal for curtailing veterans' preference at an American Legion convention in Denver last August. The legion, a key veterans' group, balked at his proposal.

Campbell said yesterday that he would now turn his efforts to Capitol Hill where he would testify on the administration's proposals and buttonhole members of Congress to press for support.

How much the administration would be willing to compromise on its package was unclear, sked whether he would agree to drop the proposal for curtailing veterans' preference if congressional opposition mounted, Campbell replied, "If I have the choice, the answer is no. I'm not sure I'll have the choice."

The proposals for paring veterans' job preference was in such apparent jeopardy that the major veterans groups already were predicting, openly or implicitly that Congress would reject the administration's proposal[WORDS ILLEGIBLE]

"Veterans' preference will not pass." Cooper T. Holt, executive director of the Veterans of Foreign Wars, said in an interview. "Each House member is up for election I think every member of Congress will take this into consideration." Austin E. Kerby, economics director for the American Legion, noted. He added that there are currently 30 million American veterans.