Senate Republican leaders said yesterday that they have tentatively agreed to a one-year freeze in Social Security benefits if they can nail down enough other spending cuts, including defense, to reduce deficits by about $50 billion next year.

There were signs yesterday that pressure for major reductions in military expenditures is building rapidly in both the Senate and the House.

In the House, half the Republicans on the Budget Committee indicated interest in pursuing a freeze of defense spending at current levels.

"We think it timely that the defense freeze option be closely examined," the six Republicans wrote in a letter to Defense Secretary Caspar W. Weinberger, who has said a freeze would "cripple" the current military buildup.

Weinberger, in an appearance yesterday before the Senate Budget Committee, read a long list of critical weapons systems that would have to be terminated or delayed and said he "could go on for four pages."

This prompted Budget Committee Chairman Pete V. Domenici (R-N.M.) to say, "I have a great deal of difficulty believing that would be the result" of a freeze in defense spending.

Senate Republicans have been looking from the start at a defense freeze or cutback and a one-year curtailment of cost-of-living increases for Social Security recipients as part of a package of spending cuts aimed at cutting deficits by half, to $100 billion by 1988, starting with cuts of about $50 billion next year.

Statements yesterday from Domenici and Senate Majority Leader Robert J. Dole (R-Kan.) indicated that the Senate leaders are ready to include the Social Security freeze in the package so long as certain conditions are met, including defense cutbacks.

"We've already addressed this in the Senate Finance Committee on the Republican side, and we've agreed to put the Social Security COLA change in our package," Dole said in remarks to a breakfast of the American Bankers Association.

"We believe we can make that Social Security adjustment without doing violence to anyone," Dole added, although he later emphasized that no decisions have been made.

Later, Domenici, whose panel is drafting specifics of the deficit-reduction plan, laid out conditions under which he would support a one-year freeze in the Social Security cost-of-living-adjustment (COLA), indicating he believed that Dole shared his view.

"I am going to support it if we can put a $50-to-$54 billion deficit-reduction package together and if it appears we have a reasonable chance of passing it," he said in answer to questions at the National Press Club.

A one-year freeze of Social Security benefits would save an estimated $6 billion next year, or $22.1 billion over three years. Savings from curtailment of all inflation adjustments for major benefit programs, including Social Security, would total $9.4 billion next year and $35.6 billion over three years.

Despite the attention given the Dole-Domenici comments, defense spending continued to dominate discussions on deficit control in both chambers.

In preparation for Weinberger's appearance later this month before the House Budget Committee, the six Republicans asked that he detail in advance the effect of implementing an overall freeze.

"The freeze is an interesting concept and we wanted to say to Weinberger , if you had a freeze, how would you spend the money?" said Rep. Willis D. Gradison Jr. (R-Ohio).

"I hope it's not a Washington Monument response. If we can't get a response , then we'll have to make our own best judgments," Gradison said.

In the Senate, Domenici sharply disputed Weinberger's contention that a military spending freeze would "cripple" the buildup.

Noting that the Pentagon claims to have received 96 percent of what it wanted over the last four years even as Congress cut $60 billion from its spending requests, Domenici observed tartly, "That's tremendous if you can cut that much and still get that proportion of what you want."

Though stopping short of supporting a freeze, Domenici told Weinberger:

"As you know, many of us are considering a freeze on overall defense appropriations . . . . I don't know whether that will come to pass, but there is a growing expectation that Congress will make significant reductions from your request."

Asked if he also would support President Reagan's proposals to eliminate such programs as the Job Corps, Economic Development Administration, Legal Services Corp. and revenue-sharing for local governments, Domenici said he would if they were part of an overall package. But he said revenue-sharing should be allowed to continue for another year, not killed next year as Reagan proposed.

"It is kind of a put-up-or-shut-up time, as I see it," he said.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), a supporter of many domestic programs, said the Democrats will look at "small business, revenue-sharing and UDAGs urban development action grants ."

Dole told the bankers that the Small Business Administration may be "cut back to some degree, but I doubt we would eliminate it."

At the Senate Budget panel hearing, Weinberger was asked about his defense of military pension programs in light of cutbacks proposed by Reagan in pensions for civilian workers. "In attempts to get hold of budget deficits," Weinberger said, "there frequently cannot be equity."

Military pensions became an issue earlier in the week after Office of Management and Budget Director David A. Stockman attacked them in a general broadside that also included criticism of farmers seeking government help with their debt problems.

Yesterday, Stockman took on another potent group: the country's colleges and universities. Speaking of Reagan's proposed cutbacks in student loans during a hearing before the House Appropriations Committee, Stockman said, "Now you're going to get a lot of pressure from the colleges . . . . They're not worried about the students, they're not worried about equity in America, they're worried about financing their budgets."