The Reagan administration, seeking to accelerate progress with the Soviet Union on a treaty reducing long-range nuclear weapons, may drop an earlier demand for sweeping reductions in Soviet warheads on land-based intercontinental ballistic missiles, senior U.S. officials said yesterday.

If President Reagan decides on this shift, it will signal U.S. willingness to let the Soviet Union decide how to distribute nuclear warheads between land-based and submarine-based missiles after a new agreement cutting overall arsenals by about 50 percent goes into effect. Until now, the administration has hoped to force the Soviets to put more warheads to sea on submarines than the Soviets have wanted.

Reagan is expected to make a decision on this matter at a meeting today with his senior arms-control and military advisers. He could inform Soviet leader Mikhail Gorbachev of any change during next week's summit here, the officials said. Statements by Reagan aides yesterday suggested that they think he will decide on the more flexible bargaining approach.

U.S. and Soviet officials have said they hope to reach accord on key aspects of a treaty on long-range, or strategic, arms during the summit, besides signing a treaty eliminating intermediate-range nuclear forces (INF) that has already been agreed to.

Arms Control and Disarmament Agency Director Kenneth L. Adelman signaled the possible U.S. shift when he told reporters yesterday, "I'm not convinced it's essential" for the United States to continue demanding that the Soviet Union reduce its warheads on land-based strategic missiles from roughly 6,400 to 3,300.

Without this demand, the U.S. position would still call for the Soviets to accept a 52 percent cut in the number of land- and sea-based intercontinental ballistic missiles, and would require a 50 percent cut in their SS18 land-based missiles, biggest and most accurate in the Soviet arsenal.

The administration has sought limits on Soviet land-based missiles because these powerful and accurate weapons pose the greatest threat to U.S. ballistic missiles in silos hardened against nuclear explosions. By altering the negotiating position now, the United States will allow the Soviets to deploy more land-based missiles, though not more of the most-formidable SS18s.

Adelman told reporters it was "preferable" to have lower limits on land-based missile warheads, but "not essential."

In previous bargaining, the Soviets have said that if the United States insists on drastic cuts in their land-based force, they would demand comparably big reductions in the U.S. force of submarine-based missiles. In October, for example, Gorbachev said his government would accept a limit of 3,300 land-based missile warheads only if the United States reduced its submarine-launched ballistic missiles from 5,640 to 2,000 warheads. The administration said this limit on U.S. submarines, backbone of the U.S. nuclear weapons arsenal, was unacceptable.

Another senior U.S. official, briefing reporters on condition that he not be identified, said Adelman's suggestion that the previous U.S. position on land-based missiles could be altered expressed "the prevailing view" in the administration and said the demand might be dropped "if we get something for it" -- in other words, if the Soviet Union offered a concession during discussions in Washington next week. Officials would not say what sort of concession they would seek.

Chief U.S. negotiator Max M. Kampelman indicated yesterday that a limit on different types of intercontinental missiles is the key issue facing Reagan and Gorbachev. Success on this point should be the "standard by which to judge our degree of satisfaction" with the Reagan-Gorbachev summit, Kampelman said.

A U.S.-Soviet compromise allowing each side the "freedom to mix" weapons has been predicted by arms-control experts outside the government because of complaints from each country that proposals to set more specific limits represented undue meddling by the other side in military decisions.

Under the new proposal being discussed in the administration, the superpowers would still be barred from exceeding a separate limit of 4,800 missile warheads on land- and sea-based forces, requiring a 52 percent cut in current Soviet forces and a 40 percent cut in U.S. forces.

Several U.S. officials indicated this total could be eased somewhat during the negotiations to accommodate Soviet desires for a slightly larger arsenal.

U.S. officials and independent experts said the Soviets were unlikely to deploy much more than 3,300 land-based missile warheads under the treaty, even if that specific limit is dropped, because they would have to give up too many existing missiles on submarines in order to remain within the overall limit of 4,800 warheads. Submarine-based missiles are seen by both sides as critical elements of their deterrent forces, since they are virtually invulnerable to enemy attack.

In another development yesterday, the Senate Appropriations Committee approved a bipartisan statement endorsing a "long-term and robust" Stategic Defense Initiative (SDI) research program in terms that seek to define it as both a defensive response to Soviet programs and as leverage to bargain for reductions in offensive strategic weapons.

The statement, aimed at finding points of agreement between Congress and the White House on SDI as Reagan prepares for his meetings with Gorbachev, stressed development of new techologies with "long-term potential," as opposed to quick-deployment systems advocated by some conservatives.

It also said future SDI funding should reflect "realistic projections of available resources" for defense and avoid undercutting resources for other military programs.

The statement was sponsored by defense subcommittee members Ted Stevens (R-Alaska), J. Bennett Johnston (D-La.) and William Proxmire (D-Wis.), who have often disagreed in the past on plans for SDI.

Staff writer Helen Dewar contributed to this report.