President Carter's turnaround on defense spending was greeted with skepticism yesterday as several Senate Armed Services Committee members suggested that the proposed increase stemmed from worries about reelection and the fate of SALT II rather than any new military threat.

"What's happened?" since Sept. 14, when Carter opposed the same percentage increase he is now requesting for defense in fiscal 1981, Sen. Henry M. Jackson (D-Wash.) asked Defense Secretary Harold Brown at yesterday's hearing.

"Does this have some relation to votes? Is it a quick fix that's going to disappear once we have voted on SALT II? We really have a right to know.It goes to credibility," Jackson said.

Carter's defense budget for fiscal 1981 will ask for $157 billion in budget authority, with spending expected to be about $142 billion. That budget will be considered next year, along with the strategic arms limitation treaty, now facing an uncertain fate in the Senate.

Although the higher defense budget was urged by several senators whose votes the president hopes to win for SALT II, yesterday's hearing failed to show whether the proposed increases had made any converts. Sen. Sam Nunn (D-Ga.), for example, said he applauded the bigger defense budget but still needed more time to study it before committing himself on SALT II.

"Would this budget be up here if SALT II was not in trouble," Jackson asked Brown as he hammered away at the administration for changing its mind about defense spending in the space of three months.

On Sept. 14 Carter wrote Sen. Ernest F. Hollings (D-S.C.) that, "As the result of other economies and improved coordination of our defense programs with those of our allies, we should be able to carry out our defense objectives without exceeding the 3 percent level of annual increase in 1981 or 1982."

Brown, in response to Jackson and other senators, said the 5.7 percent increase over fiscal 1980 that Carter decided on for fiscal 1981 "was not put together to sell SALT."

Brown told Jackson that "we don't share your judgment" that the Senate will defeat SALT II.

"The president's positon was and remains that there should not be an arbitrary increase" in defense spending, Brown said.

However, in a reference to the Iran crisis, Brown added that "we have ovserved since" the president came out against the 5 percent increase in September "that it is important to get to remote corners of the world."

Carter's new budget contains down payments for the rapid deployment force of Marines and Army troops organized to respond quickly to trouble anywhere. Funds to start building a giant cargo plane for those quick-reaction troops and the first two of a fleet of ships full of military gear for them also are in the new budget.

Other reasons for the budget increases, Brown continued, include the Soviets' continued modernization of nuclear missiles aimed at U.S. allies in Europe and the threat of the U.S. Navy from the Soviet Backfire bomber.

"All these things put together produce this result," he said. Jackson, who has long argued for higher defense spending, termed Carter's proposed fiscal 1981 defense budget "a tremendous increase."

"If you had come in with these huge increases back in 1977," Jackson told Brown, "we could be spending less for defense" under an arms control agreement with much deeper cuts in weaponry than provided in SALT II. "The thing that runs through my mind is what has happened" to change the president's mind. "Is there some new secret?" he asked.

Brown replied that the recommended increases "relate to an appreciation that we are in for a long pull of adversary relationships" with the Soviet Union and are confronted with "chaos in a large part of the world."

Carter's defense budget also received a cool reception from Sens. William S. Cohen (R-Maine) and John C. Culver (D-Iowa).Culver reminded Brown that the late Senator Richard B. Russell (D-Ga.) had taken the lead in sinking a Pentagon plan for anchoring cargo ships near likely trouble spots. Russell contended that the capability to intervene in remote conflicts would turn the United States into the world's policeman.

Brown agreed that Carter is going back to the idea of pre-positioned ships. He said the ability of the United States to respond quickly to trouble in remote areas is essential, and that the appearance of a modest force of troops at the right moment could affect the outcome of a crisis vital to U.S. interests.

In contrast to the skeptics, Nunn reacted warmly to Carter's new defense budget. He did not go so far as to commit himself to voting for SALT II as a result of the proposed increases, but he said his reaction was "positive."

Nunn told Brown "I applaud your initiative" in increasing the size of the defense budget, but noted "we're not going to have a SALT II treaty up her every year."

In a side play during Brown's questioning, several committee members tried to get Chairman John C. Stennin (D-Miss.) to set a date for voting on a report critical of the arms limitation treaty.

Jackson complained that senators had a right to vote on the report, which calls Salt II unacceptable. Sen. John G. Tower of Texas, ranking Republican on the committee, agreed, but Stennis rebuffed their efforts. "The minority has some rights under the committee rules," Tower said toward the end of the hearing.

"I second the motion," said Jackson, visibly vexed by Stennis' refusal to set a date for a vote.

Later, Stennis scheduled a committee meeting for Tuesday. Jackson, Tower and others are expected to call up the critical report for a vote then.

On other subjects:

Draft.Stennis disclosed that he had concluded that "we're going to have to have some kind of selective service to get enough of the talent that's required" for the armed services. Brown replied that the administration would have something to say on that topic in its report in January on draft registration.

Trident II missile. Brown said he remains uncommitted on whether to put this advanced submarine missile into production. The Trident would be so accurate that it could be used to destroy Soviet missiles underground. "We're reserving judgment" on whether to undertake what would be an $8 billion program, he said.

FB111 bomber. The president did not include money for this bomber version of the TFX, but did include funds to continue research on an advanced bomber to succeed the B52.

Smaller ships. Brown said that, under the new budget, the Navy would get money for a smaller and cheaper nuclear attack submarine than the $500-million-a-copy subs now being built.

Indian Ocean. Brown said the administration is considering ways to increase the American presence there, including stationing Marines on ships and sending some of the Sixth Fleet, now in the Mediterranean, to the Indian Ocean. NATO nations, under the latter concept, would fill in for any ships that left the Mediterranean.

In another development related to defense spending, the Senate yesterday approved a compromise appropriations bill that would give the Pentagon $131 billion for fiscal 1980, which began Oct. 1. The measure now goes to Carter for his signature.