The chief judges of the city's courts waited yesterday that the D.C. Superior Court will be shut down for as long as three months this summer unless Congress votes more money to keep it running.

Chief Judge Theodore R. Newman Jr. of the D.C. Court of Appeals told the Senate D.C. Appropriations subcomittee that the shutdown could start between July 1 and Aug. 15. It would halt both criminal and civil jury trials until Oct. 1, when the new fiscal year starts.

Arraignments and nonjury trials before judges could continue during the shutdown.

Despite the warning, D.C. budget director Gladys W. Mack said the city does not support the courts request for a $1.2 million supplemental appropriation, but added that city officials "are willing to discuss it."

Seeking to avert an operating deficit of up to $172 million, Mayor Marion Barry and the City Council omitted the added money for the court from a request to Congress for an additional federal payment this year of $61.8 million. The House Appropriations Committee has recommended only $28.8 million, none of it for the courts.

The possibility of a summer shutdown was raised April 2 by the Superior Court's chief judge, H. Carl Moultrie I, in testimony seeking added funds from the House.

That effort having failed, the two judges yesterday asked Sen. Patrick J. Leahy (D-Vt.) to add the funds to the final version of the bill. Leahy gave no hint of what his decision would be.

In April, the judges told the House hearing that a court shutdown affecting mostly civil cases was possible. Yesterday, with Moultrie sitting alongside him, Newman said "it is not a question of whether, but when" the lack of funds will force the Superior to halt both criminal and civil jury trials.

Moultrie, in prepared testimony not delivered orally, also predicted that court employes will have to be furloughed without pay for 8 to 10 days at the end of this fiscal year.

Acknowledging the city's financial problems, Moultrie said in the statement that "it is inconceivable to me that the administration of justice for the citizens of the District of Columbia could be permitted to be further compromised."

The threatened court shutdown is rooted, the judges said, in underfunding by the city and Congress of the basic budget for this fiscal year. Even after imposing a job freeze and a ban on overtime pay, Moultrie said the Superior Court cannot absorb $550,000 in pay raises granted last year to employes and is short $650,000 needed to pay jurors and witnesses.

Larry Polansky, executive officer of the courts, told a reporter the shift of the warning from that of a possible shutdown to an almost certainty resulted from close monitoring of the court's financial situation.

He said $5,000 needed each day to pay witnesses in criminal cases will run out in July. By August, up to $7,000 needed daily to pay jurors will be exhausted.

Although major civil cases are recessed in the summer, Polansky said criminal jury cases continue to be heard. If these are halted, Polansky said many lawyers are likely to demand future jury trials for their clients, reducing the number of guilty pleas and adding to the court's backlog.

Leahy scolded budget director Mack for the city's failure to send the court budget request to Congress.

"I can just see the headlines right now," Leahy said, predicting that if Congress goes along with Barry and refuses the added money it will be cited by city officials as "one more example of those people on (Capitol) Hill with callous disregard for the people of D.C. because we shut down their courts."

During a wide-ranging three-hour hearing, chiefly on the city's proposed budget for the 1981 fiscal year, Leahy defended Congress against criticism by city officials and citizens that it is stingy in its financial support for the District.

In the six fiscal years since home rule began in 1975, Leahy said, Congress has voted 92 percent of the aggregate authorized federal payment to the city -- a far higher proportion of its budget than most federal agencies receive, he declared.

Since 1975, Leahy said, the city was authorized to get a total of $1.664 billion, and will receive at least $1.544 billion of that figure, depending on the fate of the pending supplemental.

During the hearing, D.C. school board president R. Calvin Lockridge said Barry's proposed cuts in the school budget would have a "devastating" effect on education.

Leahy also said he wanted to meet with officials of the University of the District of Columbia to devise a final plan for campus development to be submitted for approval by the Senate Appropriations Committee this year. Adoption of a plan would release $57 million that already has been appropriated but has been held back.