Senate Armed Services Committee Chairman John G. Tower (R-Tex.) yesterday said it would be very difficult to get a proposal for an air-based MX intercontinental ballistic missile through Congress, especially since the idea "has already been studied and discarded by the experts."

President Reagan reportedly is prepared to recommend such a system in place of a land-based MX system advocated by the Carter administration but opposed by some influential western senators.

Tower said at a news conference that Defense Secretary Casper W. Weinberger called him Friday night to say that there had been no decision on the basing of the controversial MX missile, and that the land-based missile approach is still a possibility.

At the White House, spokesman David R. Gergen also said that the president has made no final decision, although Reagan continues to have reservations about a land-based MX.

Tower said he knew of no definite support at the Pentagon for the Air Mobile plan, which initially would place 100 MX missiles aboard 100 C5 transports for aerial launching, nor did he know anyone in the administration who has "come out flat-footed for it."

"I have no indication where the president is actually leaning," Tower said. He noted that the House and Senate Armed Services committees have "studied and virtually discarded" the air-based missile plan as "too unreliable, too costly and of questionable survivability."

Air Mobile is one of the three alternatives most often considered for the MX, he said. The other two are the use of submarines and land-based multiple protective shelters (MPS), he said.

Tower said he will continue to support the land-based missile approach unless new information reveals a better system, adding, "We should continue to look for alternatives, but we must not further delay our ongoing intercontinental ballistic missile program in the process."