Government lawyers at all levels have joined a sizable segment of Washington's most influential private attorneys to fight a proposal aimed at blocking the "revolving door" between government and private practice, which has come under increasing criticism for giving an unfair edge to big business in its dealings with government.

"The idea that a government lawyer should be able to enter private practice only after a year in limbo, a central feature of the draft, is preposterous," said Fritz R. Kahn, of Verner, Liipfert, Bernhard & McPherson. His view echoed the comments of other private attorneys.

The foot-high stack of comments to its December proposal still must be considered by the ethics committee before it makes its final recommendation, which then must be approved, modified or rejected by both the D.C. Bar Association's board of governors and the D.C. Court of Appeals.

The comments opposing the proposed rules show more clearly than ever how many lawyers use a job in government as a stepping stone to high salaried positions with big Washington law firms.

President Carter attacked the practice during his campaign and last May proposed an "ethics in government" bill that would curb what he called "the revolving door practice that too often permitted former officials to exploit their government contacts for private gain.

"All too often officials have come into government for a short time and then left to accept a job in private industry, where one of their primary responsibilities is to handle contacts with the former employer," the president said. His proposal has not been acted upon by either house of congress.

The bar's proposals are more far reaching than President Carter's, but they would affect only lawyers, not members of other professions or the military.

"Every lawyer presently with the federal government, should know how seriously the committee's proposal would circumscribe, if not eliminate entirely, his or her opportunity ever to leave the government and engage in private practice," wrote John A. Knebel, president of the Federal Bar Association, which represents 15,000 present or former government attorneys.

Moreover, the government's top attorneys said it is hard enough now to attract bright law school graduates into the federal service, where starting legal salaries average $15,000 to $16,000 compared to $28,000 that New York's Wall Street firms start lawyers at and the $25,000 starting salaries of Washington's top firms.

If the bar's restrictions go through, they said, attracting top lawyers into the government will be virtually impossible.

"The mere prospect that the D.C. Bar rules may be adopted has caused some outstanding candidates to decline offers of employment," the chief legal officers of the Departments of Treasury, State, Defense and Commerce wrote.

Yet, according to the January issue of the American Bar Association's magazine, Student Lawyer, thousands of law school graduates competed last fall for jobs as federal lawyers.

According to Labor Department figures, there are only 21,000 jobs available for each year's 30,000 law graduates. But, although there is a plethora of lawyers, the competition is keen for the top graduates from the top schools.

Earl J. Silbert, the U.S. attorney for the District, registered "as strong an objection as possible." He said the proposed ethics requirements would "discourage and deter top-ranking law graduates from applying to this office as well as other government agencies."

Silbert said lawyers generally join his office with two motives: "to serve the public and to gain litigating experience" that will help them get a better job in private law practice.

Even the judges of the U.S. Court of Appeals for the District of Columbia, generally regarded as the most liberal group of federal judges in the nation, opposed the regulations of the grounds it would make it hard for them to recruit law clerks.

Taking a different tack, the largely black Washington Bar Association (formed in 1925 when no blacks were allowed to join the all-white bar groups) said the proposed regulations would stifle the still embryonic entry of black lawyers -- many of whom until recently were restricted to jobs in the city or federal government -- into the large, high-paying Washington law firms.

"It will limit job opportunities to lawyers who historically have had limited job opportunities in their chosen profession," wrote J. Clay Smith Jr., board chairman of the Washington Bar Association.

Moreover, he said, it will hamper fledgling black law firms from competing with established firms because the new rules would stop them from hiring top lawyers in government.

"In recent years, because of affirmative action programs in law school and the federal government," Smith wrote, "several minorities have been exposed for the first time in the history of this nation to specialized law areas.

"In short, many minority attorneys have learned what is popularly referred to by some members of the bar as 'uptown law.' Job mobility and turnovers have created new law firms composed of minority or women lawyers or the hiring of lawyers with specialized expertise by the older black firms in order to diversify their practices other than traditional practice areas."

"The proposed rules would perpetually limit the growth and development of minority law firms to diversify into.

Besides the U.S. Attorney's office here, the D.C. Court of Appeals and the department of Treasury, State, Defense and Commerce, other letters opposing the changes came from the Nuclear Regulatory Commission, the departments of the Air Force, Army and Navy, Internal Revenue Service, Federal Trade Commission, National Labor Relations Board and Securities and Exchange Commission.

Michael J. Egan, the associate attorney general, representing the Justice Department, and Charles D. Ferris, the newly named chairman of the Federal Communications Commission, both write against the ethics rules.

Ferris, who said he has worked for the government or in politics all his life and has no intention of going into private law practice, said he feared the rules would "shut down any movement of lawyers to and from government service."

Lawyers from the Institute for Public Interest Representation of the Georgetown University Law Center described the proposals as "excessively draconian."

A group of 37 lawyers from the firm of Fried, Frank, Harris, Shriver & Kampleman said some provisions "would basically compel a D.C. lawyer to choose between public service which may alternate only with private practice having a nongovernmental orientation, and a government-oriented private practice."

Other lawyers pointed out that the disciplinary regulations would be unfair to Washington lawyers since attorneys based elsewhere would not be governed by the D.C. Bar ruling and could therefore use the "revolving door" with impunity.

"By choosing to set up practice only a few miles away in the Virginia or Maryland suburbs, a former government lawyer could avoid the disqualification which he would otherwise incur if he practiced here," wrote Michael F. Curtin, chairman of the legal ethics committee of the Bar Association of the District of Columbia, a private voluntary group as distinguished from the D.C. Bar to which all lawyers here must belong and which has disciplinary powers conferred on it by the courts.

In his letter, Eugene J. Metzger of Metzger, Shadyac & Schwarz provided graphic evidence of how the "revolving door" works:

"Our practice, like that so many of our colleagues in the District of Columbia, is highly specialized. We appear before a limited number of agencies where most of our partners now served," he wrote.

"The people with whom we deal could be effectively precluded by your regulations from ever joining us as we would be precluded from returning to those agencies by the same rule.

"To what end? Is there any evidence, beyond mere suspicion, that the so-called revolving door has been significantly abused? We have seen none."