While President Reagan stoutly resists negotiating any limits with the Soviet Union on his Strategic Defense Initiative (SDI), the president's timetable for the "Star Wars" program has nevertheless been substantially eroded by congressional budget cuts, including delays of up to two years in experiments on controversial space-based weapons, according to Defense Department documents.

At the rate that Congress has been appropriating money for SDI, the program will receive barely more than half of the $26 billion the administration wanted by fiscal 1989, according to congressional sources.

In the long run, the cuts could affect some of the most fundamental decisions about Star Wars, including whether a defense against enemy warheads should be primarily based in space or on the ground. These changes are being made as some SDI scientists and officials are looking more favorably at ground-based systems.

Experiments with exotic space-based weapons originally scheduled for as early as 1991 are being pushed back further into the 1990s. This part of the program has drawn the most fire from Soviet officials and American critics who say these tests may violate the 1972 Antiballistic Missile (ABM) Treaty.

"One of the laser programs that we have regrettably had to delay in a very significant way is work on the space-based laser concept," Lt. Gen. James A. Abrahamson, director of the SDI program, told a Senate Armed Services subcommittee last month. "We are more fund-limited than we are treaty-limited."

A "talking points" document distributed on Capitol Hill by the Strategic Defense Initiative Organization, the administration office coordinating the SDI program, says that the $1 billion reduction approved by Congress in the fiscal 1986 SDI authorization budget "postpones by six months to a year" resolution of key technical issues on "boost phase engagement," the crucial question of how to destroy enemy missiles shortly after they leave their silos.

In addition, major experiments on kinetic weapons "will be delayed up to approximately one year," the paper says. For example, in the wake of the funding cuts, the Air Force has recommended a 28-month delay in experimental flights of the space-based "kinetic kill vehicle," sometimes called a "smart rock," a projectile that would be fired at missiles during the boost phase.

Abrahamson told the senators that "it is too early to speculate on the kinds of defense, whether ground-based or space-based, and the best merits of each." However, he went on to herald progress in research on directed energy weapons "where it appears that the potential for large, effective ground-based lasers is very real."

Tests of ground-based lasers, already undertaken by the Soviets, are not barred by the 1972 treaty as long as they are done at a treaty-approved test site.

In a closed session Wednesday, the Senate subcommittee was given a briefing on the newest approach SDI officials are studying for a "layered" defense system.

Originally, Abrahamson had described a five-tiered system beginning with "boost phase intercept" intended to blast Soviet missiles as they are launched. In theory, subsequent tiers later in the attack would destroy missiles or their warheads that eluded the boost phase defense.

As described Wednesday, sources said, SDI is studying a seven-layer approach. The two additional stages would be inserted during the phase of an attack when the missiles are in space and ejecting their warheads toward different targets.

Further reductions in SDI funding are almost certain to continue in the coming years, one senior congressional aide said. The SDI spending plan calls for $4.9 billion in fiscal 1987, which would be an increase of $2.2 billion over this year's current level.

That is more money than Congress is likely to approve, the aide said, noting that "as the Pentagon has to cut back on programs that are already producing weapons, SDI is trying to grow 100 percent . . . to prove principles for some weapons in the distant future."

The SDI paper warns that any further reductions this year would "require a major deviation in the program and significantly delay completion."

In testimony before the Senate subcommittee, Abrahamson said he still hoped to make "an informed full-scale engineering development decision in the early 1990s." But he said "budget cuts have caused major, and I would emphasize major, revisions in our program."

Pentagon aides have said privately that White House refusal to entertain any SDI limits in the Geneva arms talks stems partly from a belief that Congress would slash the program deeper "if Congress thought it would never be deployed," as one official said.

On Capitol Hill, however, several legislators said that if the president fails to use SDI as a bargaining chip with Moscow, and thereby loses the opportunity for deep reductions in the superpowers' offensive nuclear forces, Star Wars probably will face even deeper reductions.

"SDI support will change radically if the summit doesn't work," one House expert said last week. "Lots of its support comes from members who do not think it is worth much, except as a bargaining chip."

Reagan and Defense Secretary Caspar W. Weinberger have said that no deep cuts would be worth trading away the possibility of finding a missile defense that could make "nuclear weapons obsolete."

Consequently, U.S. negotiators have made clear in Geneva, Moscow and Washington that SDI research, including experiments and even flight tests of devices outside the laboratory, are not negotiable. But as Paul H. Nitze, the president's special adviser on arms control, told a House subcommittee, "everything else" involving SDI is negotiable, presumably including the construction of such a defense.

Some former U.S. negotiators think that both sides are delaying any attempt to compromise on futuristic space weapons until they see what kind of deal can be worked out on nuclear weapons that exist or are under construction.