The Air Force was fighting yesterday to keep a potentially damaging letter about a new MX basing plan from being sent to the White House when Defense Secretary Caspar W. Weinberger submits his recommendations to President Reagan, probably today.

The letter, according to Pentagon sources, was written to Weinberger on Sept. 22 by Charles H. Townes, the Nobel Prize-winning physics professor from the University of California who headed a study group Weinberger set up to examine the new plan.

The Air Force has had enormous problems the last few years finding a militarily safe and politically acceptable home for the MX. The new plan, called Dense Pack, involves bunching the new missiles. The theory is that the blast from the first attacking enemy missiles would deflect or destroy the trailing missiles and leave the MXs relatively secure.

The letter reportedly expresses doubts as to whether the Air Force can build the kind of superhardened underground concrete and steel silos needed to protect the MXs from the blast, heat and radiation of an attack.

It also reiterates Townes' view that it may be less difficult technically for the Soviets to develop a counter to Dense Pack than it will be for the Air Force to build an MX system as now envisioned.

Weinberger returned to Washington last night after a trip to the western Pacific.

Sources said that last night the secretary's recommendation was for deployment of 100 MX missiles in 100 of the new superhard silos. The silos would be spaced close together in a 14-mile row.

Weinberger, however, has always kept his final weapons decisions closely guarded, and sources said there remained the possibility he would order a last-minute change.

Along with Weinberger's recommendation, several other documents will go to the White House, sources say. These include the basic Air Force recommendation supporting Dense Pack and an Air Force study on where to put it.

Bases in New Mexico, Nevada and Wyoming were all cited as acceptable by the Air Force, although New Mexico is said to be best. Sources say at the moment it does not appear that Weinberger will make a specific recommendation on location and could leave that politically touchy choice to the White House.

The Pentagon will also include a memo on the legal implications of Dense Pack for the unratified but informally observed U.S.-Soviet SALT II agreement, the treaty with Moscow limiting antiballistic missile (ABM) defense, and U.S. environmental laws.

The analysis reportedly concludes there would be no violation of the SALT II agreement because the MX canister would be portrayed as a mobile missile launcher and thus not a violation of the restriction on new underground launchers. But there is no chance the Soviets will accept such an interpretation, Pentagon officials acknowledge.

Although Weinberger is not expected to recommend that the United States build a new ABM to help protect Dense Pack, his recommendation reportedly calls for stepped-up research so one could be built quickly if necessary. This, officials say, almost certainly would require changes in the ABM treaty.

The package also includes a memo from the Defense Intelligence Agency supporting the viability of Dense Pack against the Soviet threat, a copy of the Defense Science Board report also giving a qualified endorsement to Dense Pack and, at the moment, the Townes letter.

The concern over the Townes letter at top U.S. Air Force levels is over several issues.

Townes headed the Defense Science Board (DSB), which reported to Weinberger this fall that the Soviets could not have confidence attacking Dense Pack with their current missiles or those now thought to be planned. The board said this vote of confidence was based on the proviso that the Air Force can achieve the desired silo hardness.

The Air Force was pleased by this DSB report. But now Townes has gone beyond it, in what the Air Force views as a personal capacity.

Townes' opinion is known to be highly valued by Weinberger. And, even though Weinberger is said to be ready to recommend a go-ahead on Dense Pack to the White House, the defense chief reportedly remains "lukewarm" at best about Dense Pack and has incorporated Townes' concerns into his official recommendation. The Air Force is trying at least to get the basic Townes letter out of the material going to Reagan.

In his letter, Townes reportedly notes that the DSB said the extra silo hardness is feasible. But he cautions that this is without any clear limits on time to achieve this or expense. He notes that for the system to work, a degree of hardness would have to be achieved substantially beyond anything done before and that the Soviets must believe we can do it.

Townes cites "troublesome possibilities," sources said, that the United States could miss these hardness goals, that the difficulties in reaching them could delay MX deployment and that significant cost overruns could result.

The Air Force points out that its analysis and that of the Defense Nuclear Agency and other Pentagon scientists support the idea that the desired level of protection can be achieved.

The root of the problem, Townes is also said to have observed, is that it may be easier for the Russians to modify their missiles to cope with Dense Pack by the end of this decade than it is for this country to build such a superhardened shelter.