U.S. Air Force leaders are hoping to use the strategic arms limitation treaty to create a political backfire of an American version of the controversial Soviet "Backfire" bomber.

Although the White House and Defense Secretary Harold Brown are publicly neutral on a hotly lobbied proposal to turn the F111 into an intercontinental bomber, the idea is being pushed strongly by the Strategic Air Command and its allies in Congress.

The SAC plan would give the fighter-bomber, orginally conceived in the early 1960s as the TFX, a strategic capability of reaching the Soviet Union from American soil.

But by taking advantage of the SALT II loophole created for the Soviet Backfire, the aircraft would not be counted as a strategic weapon subject to the treaty's weapons limitations.

In Senate testimony given in closed session earlier this year and released last week, Gen. Richard H. Ellis, SAC commander, declared the modified FB111B "would effectively offset the ommission of the Backfire from SALT II."

SALT II opponents want the treaty changed to require that the Backfire be included as a strategic weapon because, they argue, it could reach some parts of the United States from Russia.

At Vienna, after a heated exchange, Soviet President Leonid Brezhnev gave his verbal assurance that only 30 Backfires a year would be built. In a written statement, Brezhnev said Backfire was "a medium-range bomber" that will not given "the capability of operating at intercontinental distances."

Until the treaty was signed, the Joint Chiefs of Staff consistently argued that the Backfire must be covered by the agreement.

The SAC proposal may be introduced in the Senate as an amendment to the fiscal 1980 defense authorization bill, according to aides of Sen. John G. Tower (R-Tex.), who supports the plan and has indicated his opposition to the SALT II agreements.

The SAC proposal thus creates a potential dilemma for the president.

If he turns down the idea, he would seem to be backing away from a statement made before the Vienna summit that the United States, in allowing the Soviets to keep Backfire out of SALT II, retained the right to build for itself a comparable bomber.

If he approves the new program, liberal critics will say he is buying a new, unnecessary weapon to gather Senate votes for SALT ratification.

Asked last week if he would support the FB111B modification program, Defense Secretary Brown said, "I still need a lot of convincing."

SAC Commander Ellis told the Senatre committee that the first modifiedFB111Bs could be ready 29 months after the program began. It would take slightly more than three years to have 30 bombers ready for service.

The quick upgrading of the F111 is needed, according to Ellis, to generate a larger retaliatory force in the early 1980s when, according to the administration's estimates, U.S. land-based ICBMs theoretically will be vulnerable to a Soviet first strike.

"For a fairly reasonable investment," Ellis said, "these modified F111 aircraft would give our force more range, more mass and greater damage potential by adding the capability to deliver [number deleted] more warheads on target."

The SAC proposal was presented to the Pentagon earlier this year for inclusion in the fiscal 1981 budget, which is in the early stages of preparation.

It contemplates modifying 89 of the Tactical Air Command's F111D fighter-bombers and 66 FB111A's, which belong to the Strategic Air Command.

Were the United States to go ahead with the FB111B program, it could create for the Soviets more of a military threat than the Backfire poses to the United States.

Backfire, as now constructed, had no refueling capabilities. Therefore, any mission to the United States would not permit it to return to its home base.The Soviets also currently are keeping the Backfires at airfields that place them out of U.S. range.

In addition, half the Backfires now produced are configured for antiship warfare and are turned over to the air arm of the Soviet navy.

The other half, according to the Soviets, are used to provide nuclear air support for Warsaw Pact forces in Europe.

The proposed FB111B, by contrast, could be refuled and thus could reach anywhere in the Soviet Union and still return to the United Sates.

Furthermore, SAC specifically plans to give it the strategic role of a bomber capable of penetrating Soviet air defenses at low altitudes.