White House chief of staff Donald T. Regan told Senate Republican leaders yesterday that the Reagan administration wants protections against big, across-the-board defense spending cutbacks to be written into balanced-budget legislation working its way through Congress.

Regan said the administration objects to a tentative House-Senate agreement under which spending cuts would come equally from defense and domestic programs, and it wants more flexibility in deciding where military cutbacks would be made, at least this year.

"We're quite concerned, and we're doing our best to work with the House-Senate conferees to see what can be done," Regan said after meeting with Senate Majority Leader Robert J. Dole (R-Kan.) and Senate Budget Committee Chairman Pete V. Domenici (R-N.M.).

While the administration continues to support the purpose of the legislation, Regan said, it does not want the country's "defense position crippled" by the bill. "We have to remain strong, especially in regard to negotiations with the Soviets," he added.

Dole and Domenici had no immediate response, but Sen. Phil Gramm (R-Tex.), who cosponsored the balanced-budget legislation with Sens. Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.), said he thought "some degree of flexibility is reasonable."

Gramm said he was working with House Armed Services Chairman Les Aspin (D-Wis.) on language that would give both the administration and Congress some discretion in how defense spending cuts would be apportioned among specific military programs.

House Republican leaders were also described as sympathetic to the administration position. "We've been trying to work it out, but the Senate leadership doesn't seem to want to go along," one House GOP aide said.

A Senate source noted that its GOP leaders have held the White House at arm's length in the negotiations and are reluctant to give special treatment to any program, including defense. If the White House proposal "is perceived to make it easier on defense, it won't fly," the source said.

Regan did not say whether administration support for the Gramm-Rudman-Hollings bill would be in jeopardy if the changes are not made. Dole said he thinks that the White House "wants to support" the legislation but is "nervous" over the implications for defense.

Defense Secretary Caspar W. Weinberger has voiced strong objections to the bill, and the White House has indicated increasing misgivings over the past month about the direction the legislation is taking, as modifications are made to overcome congressional objections.

The bill would set fixed ceilings to reduce deficits from roughly $200 billion to zero over the next five years, and would require across-the-board cuts in most programs to meet the targets if Congress fell short of the mark.

The measure has been passed in different forms by both houses, and conferees are reportedly close to agreement on a compromise, including a 50-50 split between defense and domestic cuts under any automatic cutback order. The bill is tied to a debt-ceiling extension with a Dec. 12 deadline.

The major obstacle to agreement is reported to be whether Medicare would be exempted from cutbacks, along with Social Security and several poverty programs. The Senate wants to get some savings from Medicare; the House does not.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said yesterday that he hoped a final conference agreement could be reached by the end of the week but he indicated that the House will hold firm on Medicare. "We know we can't go as far as they [the Senate] want us to, no way," he said.