Three former attorneys general and the president of the American Bar Association warned yesterday that a bill to virtually eliminate court-ordered busing for school desegregation is unconstitutional and could be the first step toward a major erosion of constitutional rights.

The bill, which passed the Senate 57 to 37 in March, would prohibit federal courts from ordering that students be bused more than five miles or 15 minutes from their homes for racial reasons. It contains the strongest antibusing language ever enacted by either house of Congress.

It would also bar the Justice Department from asking federal judges to use busing as a desegregation remedy and would permit the department to go to court to overturn previous busing orders.

If the legislation is enacted and upheld by the courts, David R. Brink, president of the ABA, said, "Each Congress, according to its mood, may remove more and more of the jurisdiction of the federal judiciary until it is gone.

"Once the door is opened, another Congress, another day, next could abolish or curtail the rights of free speech, free press, free assembly or free exercise of religion or the other great protections each of us enjoys under our Constitution."

Benjamin R. Civiletti, attorney general under President Carter, agreed. "This is the first of a series of direct attacks on the independence and separate authority of the federal judiciary," he said. "It would deprive the courts of the ability to order the only effective remedy in many school desegregation cases."

Nicholas deB. Katzenbach, attorney general under President Johnson, said, "We all have to recognize that decisions of courts are not always popular. I don't think the decisions of the courts are always right. . . . But I think the system has served us well, and this is an effort to tamper with that system.

"This is an effort to get rid of busing whether or not busing is essential to the realization of a constitutional right," he said.

Elliot L. Richardson, who served in President Nixon's Cabinet, said the proposal "would disturb the balance of power between state and federal governments," and would "distort the separation of powers required by the Constitution by encroaching on the judiciary's independence."

All three former attorneys general said they believe Congress will be exceeding its allowed powers if it enacts the measure.

House Judiciary Committee Chairman Peter W. Rodino Jr. (D-N.J.) and Rep. Robert W. Kastenmeier (D-Wis.), chairman of the subcommittee with jurisdiction on the bill, are strongly opposed to its passage.

Rep. W. Henson Moore (R-La.) has filed a petition to force the Judiciary Committee to move the measure to the floor for a vote, but so far it has only attracted about 35 of the required 218 signatures.

A second piece of antibusing legislation, a constitutional amendment filed more than 18 months ago by Rep. Ronald M. Mottl (D-Ohio), is still being held in the committee. It would bar federal courts from requiring that any student be assigned to or excluded from any particular school on the basis of race, religion or national origin.

A petition to bring that legislation to the floor was filed Jan. 5, 1981, when Mottl first submitted the measure. The petition has 210 signatures, and congressional aides say they believe it is unlikely the bill will come to the floor in an election year.