Senate Republicans got off to a shaky start yesterday in drafting an alternative to President Reagan's budget for 1986 as defense advocates, led by Armed Services Committee Chairman Barry Goldwater (R-Ariz.), claimed that a spending freeze could jeopardize national security.

Senate Majority Leader Robert J. Dole (R-Kan.) said after an inconclusive two-hour debate that a comprehensive freeze remains "probable" for inclusion in the massive deficit-reduction plan.

But objections by Goldwater and Sen. Ted Stevens (R-Alaska), chairman of the defense appropriations subcommittee, clouded prospects for such a freeze, which would account for two-thirds of the senators' plan to reduce deficits by more than half within three years.

Reducing the deficit to no more than $98 billion by fiscal 1988 is the goal of both the Reagan administration and the Senate Republican leaders, who took over after the White House, reluctant to propose deep cuts in defense or tamper with Social Security, fell short of the target.

A set of options prepared by the Senate Budget Committee staff and outlined in the closed-door session by Committee Chairman Pete V. Domenici (R-N.M.) would freeze spending authority for one year for nearly all programs, including defense and Social Security.

The only exception would be programs targeted specifically at poor people, which would be allowed to expand to accommodate inflation. These would add little to projected deficits, and many lawmakers want them because of previous deep cuts.

The options also included about 40 "major program reforms" and another 40 "less desirable reforms," embracing anticipated Reagan cuts as well as some added by the Budget Committee.

Together, the options provide more than enough to reduce the deficit to $78 billion within three years, although many specific proposals for cuts are so controversial they are not expected to be enacted.

Goldwater not only raised objections during the meeting but issued a statement afterward saying that a defense-spending freeze "could seriously damage the national security of the United States and compromise our ability to provide program management stability in the Pentagon."

Even if the "top 25 weapons programs" were canceled the Pentagon could not achieve the required savings, Goldwater said, adding: "We obviously cannot cancel all of these programs without devastating our national security, but it illustrates how dangerous a defense spending freeze would be."

But, in remarks to reporters, the chairman of the armed services panel said he would want defense included in a freeze if one is adopted for other programs. Otherwise, he said, Congress might "shoot the hell out of it," apparently meaning it could make even deeper cuts in defense than a freeze would entail.

His preference, Goldwater said, would be a defense-spending increase of 3 to 4 percent, which is roughly what Congress approved last year.

Stevens, according to his colleagues, suggested that the proposed defense savings for next year be cut from $20 billion to $10 billion.

As outlined in administration documents, defense would account for $20 billion of $38 billion in anticipated savings from a freeze next year. Over three years, it would account for $106 billion of $180 billion in overall savings from a freeze.

A one-year cancellation of cost-of-living increases in Social Security benefits would save about $6 billion next year and a total of $22.5 billion over three years.

After the meeting, Domenici made an impassioned plea for consideration of a Social Security freeze if further cuts are to be made in programs for the poor. "That's not only courage; that's good policy," he said.

Domenici's remarks about courage were inspired in part by an earlier Goldwater charge that Congress lacks "guts" to make the right spending cuts. "We're dealing in guts this year," Domenici retorted.

No decisions were made at the meeting of Senate Republican leaders and committee chairmen, who will now go back to their members to explore the feasibility of the freeze and further proposed cuts.

Dole indicated that the leadership group probably will not meet again until after Reagan's Jan. 21 inauguration, leaving only 10 days for completion of the Senate draft if Republicans are to meet their target of Feb. 1, four days before the scheduled submission of Reagan's budget to Congress.

Even in advance of yesterday's session, Dole indicated that the job would be tough. "These are tough options," he said. "They're no gimmicks. There aren't any painless ways of achieving deficit reductions."