Defense Secretary Caspar W. Weinberger yesterday strongly defended President Reagan's recent claim of Soviet nuclear superiority over the United States, in the face of considerable bipartisan skepticism by the Senate Foreign Relations Committee on this and other issues central to the nuclear arms race.

Members of the panel also voiced doubts about how much public consensus remains for the high level of defense spending the administration says is necessary to close the gap with Moscow and about the tactics used by the administration to keep that spending level high.

Weinberger and Gen. David C. Jones, chairman of the Joint Chiefs of Staff, appeared at the first of what the committee chairman, Sen. Charles H. Percy (R-Ill.), called "a landmark series of hearings" on U.S. global strategy, arms control and the crucial question of how best to avoid nuclear war.

Percy said the hearings "represent the response of a democratic government to the demands and interests of the people," who, he said, are speaking up on nuclear arms control in a nationwide grass-roots movement demanding that "the government move at once to halt the deadly and destabilizing competition in nuclear arms."

Weinberger said that Reagan wants "genuine arms control" with deep reductions in the nuclear arsenals of both superpowers, and "in the near future . . . will make far-reaching proposals . . . and invite the Soviet Union to commence strategic arms reduction talks."

The Pentagon chief maintained that the last year and a half has not been wasted even though there were no talks. The time was needed, he said, to send an "unmistakable" signal to Moscow, through the defense budget, that the United States is determined to improve its nuclear forces and thus provide the only incentive that Weinberger thinks will get Moscow to negotiate seriously.

He said that the United States had not lost its ability to deter attack but that the trends were going against this country. He also said the United States does not seek nuclear superiority, does not think a nuclear war is winnable and, under questioning, that he doesn't think a limited nuclear war could be controlled, although "nobody can really say."

With the exception of a proposal by Sens. Henry M. Jackson (D-Wash.) and John W. Warner (R-Va.), Weinberger rejects the nuclear freeze proposals now on Capitol Hill, arguing that they would certify existing Soviet advantages in missiles and overall destructive power, and noting that the ruling West German political party just rejected a freeze in Europe, where the Soviets also have a missile edge.

At the heart of Weinberger's argument, however, is the balance of power between the superpowers. Percy claimed that the president's March 31 statement that the Soviets have "a definite margin of superiority" had caused considerable confusion and that "a number of distinguished experts disagree."

Percy noted that Weinberger testified previously that the Soviets had only "begun to build an edge of superiority." Sen. Nancy L. Kassebaum (R-Kan.) asked, "Who can we believe?"

Sen. Christopher J. Dodd (D-Conn.) read from Weinberger's annual report to Congress in February, in which he said the United States would make every effort "to prevent the Soviet Union from acquiring such superiority." Dodd said that any normal reading of that would lead one to conclude that the Soviets had not yet achieved nuclear superiority.

"What the president said about Soviet superiority may wash in the countryside," Sen. Paul E. Tsongas (D-Mass.) said, "but not with people who have some experience."

"It should wash and it does wash," Weinberger shot back, "because unfortunately it is true."

Weinberger claimed the Soviets were "absolutely unrelenting" in improving the accuracy, explosive power and modernization of their missiles. He talked about Soviet efforts to protect their missiles and command posts, improve civil defense and add re-fire capability to their missile silos.

While the United States still has an edge in numbers of individual atomic warheads, Weinberger said, that could disappear by the mid-1980s. He claimed that the U.S. warhead edge was not enough of a balance because what counts in deterring an attack is how many warheads would be available after absorbing a first strike. Jones noted that, in the freeze proposals, the Soviets would be able to build air defenses while the United States would not be able to build up a bomber force, which this country currently relies on.

Jones, however, provided an assessment that seemed more attuned to what the senators thought was the case. Asked by Percy if he would swap forces with the Soviets, Jones said he would not trade American bombers or missile-carrying submarines for Soviet counterparts but he would trade for Soviet land-based missiles, or ICBMs, which, he said, "are much more capable than ours."

Sen. Charles McC. Mathias Jr. (R-Md.) told Weinberger that his and the president's claims about the Russians could jeopardize support for a higher defense budget by creating a climate of fear and discouraging both Americans and allies. "There's no point in concealing the facts," Weinberger answered. "We need public support and it's going to take sacrifice and many years."

Kassebaum told Weinberger that after a trip home to Kansas she thinks the consensus for a larger defense budget is shrinking, and Sen. Larry Pressler (R-S.D.) said he was "amazed at the number of political conservatives in my area" who are worried about high defense and foreign spending.

Jones also made a strong pitch for arms control and said he "shared the concern of many people in this country." He applauded the effort to continue abiding by the provisions of SALT II even though the strategic arms control treaty was never ratified, but he said it was best now to go on to try and achieve actual reductions.

Several lawmakers are pushing resolutions to resubmit the SALT II treaty, signed in the Carter administration but bitterly denounced by the Reagan administration. Yesterday, Reps. Thomas J. Downey (D-N.Y.) and Jim Leach (R-Iowa) introduced a resolution to put the treaty to a majority vote in Congress, arguing that the pact restricts the Soviets and not this country and that "there is no reason in the world why we shouldn't ratify it."