Doris W. Johnson says she is underpaid and intimidated by her employer, who has refused for more than three years to permit the $11,700-a-year cafeteria cook and her coworkers to organize a union.

"They'll get rid of you if you try to have a union," Johnson says. "I'd like to bargain like everyone else."

Under the laws passed by Congress, most employers who deny a worker's right to unionize can be held in contempt of court or even jailed. But Johnson's boss isn't going to jail, because Johnson's boss is Congress, which has exempted itself from the very laws that guarantee nearly all other workers the right to join unions.

"This place should not be the last plantation in America," says Sen. Patrick J. Leahy (D-Vt.), who has introduced bills that would allow Johnson and her fellow Senate cafeteria workers to organize. "Presently, it is the last plantation in America."

The workers, who are predominantly black and Hispanic and who formed the Capitol Employees Organizing Group in 1979, charge that the Senate has repeatedly trampled on their rights, ignoring efforts to discuss grievances and making it clear that unions are not wanted on Capitol Hill. Congressional exemptions to various laws are justified on the grounds that they ensure the separation of powers set forth in the Constitution.

The Senate cafeteria workers' fight to unionize began three years ago, when 130 of the 223 people employed by the Senate's restaurants signed up for a union, only to be told that Congress would deal with them individually.

Rebuffed by the Senate, the organizing group appealed last year to the International Labor Organization (ILO), a United Nations body that intercedes with governments to protect workers' rights. In November, the ILO sided with the employes, saying it hopes that dialogue between Congress and the workers will permit them to "carry out normal trade union activities in full conformity with the principles of freedom of association."

That has put the U.S. government in something of an anomalous position, because the ILO governing board includes representatives of the U.S., which pays a quarter of ILO's two-year budget of $230 million.

On Friday, the ILO board members--including the U.S. representatives--determined that they will decide in November what further action, if any, should be taken. In those same deliberations in Geneva, the union group points out, the ILO board decided to look into charges that the Polish government is suppressing Polish workers' attempts to organize.

"What makes this case interesting is the substance of the situation is not under our control" because Congress decides what laws it will follow, says David Weisz, a Labor Department official who helped transmit the Senate's position to the ILO.

For the employes who serve hamburgers, tuna sandwiches and tacos each day to senators, their staff members and the public, the legal details are largely beside the point. "We need someone to talk to," says William J. Harris, president of the union organizing group. "There is no communication."

Stuart S. Smith, a government public relations officer who has helped organize the group, says, "The greatest grievance is the lack of respect. These people are not allowed to meet with their employer as a group."

Besides the issue of salaries, many of the workers who serve $4.5 million in meals each year in the Capitol and Senate office buildings assert that they are subject to arbitrary firings, and that promotions are based on race.

"The whites come in and are moved up" into management, says Dorothy M. Garnett, treasurer of the union group.

David Christianson, a trashman, says he favors a union but prefers not to discuss it. "My job might be in jeopardy if I talk about the union," he says.

Not all the workers are unhappy. "It's okay for me," says Yolanda Ramos, who came to this country from Guatemala 12 years ago and has been cleaning Senate tables for five years. "You get sick leave and annual leave."

In the Senate's arcane organizational hierarchy, the ultimate responsibility for the operation of the restaurants lies with the Senate Rules and Administration Committee, whose chairman is Sen. Charles McC. Mathias Jr. (R-Md.). The committee recently agreed to study whether to keep things as they are, grant the employes "limited" organizing rights or hire a private contractor to run the dining rooms.

Members of the committee, including Mathias, were unavailable for comment.

In March, 112 cafeteria employes signed petitions saying they do not want a union, but Matthew P. Jaffe, who is studying the issue for the rules committee, says, "It could be because they were afraid of contracting out" of the cafeteria services.

William F. Raines, administrative assistant to the Architect of the Capitol, who has direct responsibility for the dining rooms, says his office has no authority to permit unions. "The architect has nothing against organizing," Raines says. "He's just without authority . . . I don't think anybody has felt their job is in jeopardy."

With the exception of employes of the Central Intelligence Agency and other security agencies, federal workers are covered by the Civil Service Reform Act, which permits them to organize unions that bargain with the federal government.

In exempting itself from that law and others, Congress took the position that "you can't have the president controlling or regulating the legislature," says Ben C. Wimberly, general counsel to the architect, referring to the legislative branch of government, which passes laws, and the executive branch of government, which enforces them. He says Congress often adheres voluntarily to the standards in laws it passes for others.

Smith, the executive director of the union group, says the rationale given for barring union activity in the cafeterias does not explain why Congress specifically permits it at the Library of Congress, also a part of the legislative branch.

Leahy, who has introduced bills to do away with several exemptions Congress has carved out for itself, says he has been given no reason for their resounding defeat.

"People have made it clear it's not going to go anywhere," he says of the latest such bill. "We, because of the pureness of our souls, don't need the protection given the average person."