An Air Force draft report to Congress, which says 500 Midgetman mobile nuclear missiles deployed on five military bases could help counter the Soviet missile threat, has been stalled on Defense Secretary Caspar W. Weinberger's desk because of Pentagon uncertainty over the next generation of U.S. strategic forces, according to congressional sources.

A Pentagon spokesman said the so-called Midgetman tradeoff report, which originally was due on Capitol Hill two months ago, "is in final review" and an extension has been requested "to get as much information as possible."

However, one key member of the House Armed Services Committee attributed the delay to some Defense Department civilians who see the mobile missile as a competitor for funds that otherwise would go to President Reagan's Strategic Defense Initiative (SDI), the "Star Wars" research program.

His description of a Midgetman-versus-SDI clash is only one facet of a broader battle in which the Reagan administration's strategic modernization program is facing the twin hurdles of a U.S. budget crunch and the summit agreement between Reagan and Soviet leader Mikhail Gorbachev to seek 50 percent cuts in their nuclear forces.

A Reagan administration official characterized the Pentagon's present strategic program posture "as similar to rowing a boat with only one oar."

A defense aide to a Republican senator said that reluctance to send the Midgetman study forward reflects "the chaos in the president's strategic modernization program." He said the Pentagon is "still reeling" from Congress' decision earlier this year to build only 50 MX intercontinental ballistic missiles -- half the number sought by the administration.

With next year's defense budget likely to show little growth beyond this year's level, the Pentagon is faced with choosing among a series of high-profile strategic programs requiring large funding increases.

Ending the B1 bomber program in fiscal 1986 will free $5.6 billion dedicated to strategic weapons in fiscal 1987, one congressional expert said. But that money must be spread among Midgetman, SDI, the Navy's new Trident II submarine-launched missile, the Stealth advanced technology bomber and the advanced air-launched cruise missile program, which also is to have "stealth" characteristics that make it difficult to detect with radar.

Weinberger has promised House Armed Services Committee Chairman Les Aspin (D-Wis.) that he will not seek more than 50 MX missiles in silos without first devising a new method for basing the missiles that makes them less vulnerable to Soviet attack. Although the Air Force continues to hope for 100 of the giant, 10-warhead missiles, any further request awaits the result of research into "superhardening" silos to protect them from nuclear detonations and a new, mobile concept called "carry hard."

The latter idea involves placing the missile and launcher inside a cement capsule and hauling it by truck among silos filled with water. The hardened capsule is put into a silo -- although Soviet targeting strategists will not know which one. The water is then pumped into the transporter, making it as heavy when it leaves as when it arrived carrying the capsule, and further confusing the Soviets.

The system, which is being explored by the Air Force Ballistic Missile Office, is described as a "more elegant version" of the Carter administration's multiple shelter program that was dropped by Reagan in 1981. Because the capsule is hardened, it does not need the space required by the Carter system and could be based on military reservations.

Aspin said it "might work better with Midgetman than MX."

The Capitol Hill fight over MX, the focus of defense debates the past three years, probably will be replaced next year by a more modest Midgetman-versus-SDI clash.

The Air Force Midgetman study says the Soviets would have to use nearly all of their large ICBM warheads to destroy the hardened transporters, with their 500 missiles, as they travel across five unspecified military reservations, according to informed sources.

Midgetman until now has never been popular in the Pentagon. It initially emerged in the report of a commission headed by retired general Brent Scowcroft and became part of a White House compromise with Congress in 1983 to get funding for the first production of what was to be 100 MX ICBMs.

But neither the Air Force nor the civilians around Weinberger were happy with the proposed single-warhead missile because it was not as capable of destroying Soviet missile silos and cost far more to operate than the MX.

Last month, on the eve of the summit, Pentagon civilians led by Assistant Defense Secretary Richard N. Perle hastily got administration approval for an arms control proposal to the Soviets that showed their distaste for Midgetman.

It called for a ban on mobile intercontinental missiles. This would include the Midgetman, which is not expected to be operational until at least the early 1990s, and the Soviet SS25 single-warhead road-mobile missile, which is already deployed. It also would affect the Soviet 10-warhead SS24, which has been tested and is expected to be deployed beginning next year, first in silos and later in a railroad-mobile version.