Although President Reagan still hasn't decided what to do with the Air Force's controversial new MX missile, the weapon is already blowing holes in the ranks of conservative Republicans.

Yesterday, Sen. John G. Tower, the Texas Republican who heads the Senate Armed Services Committee, criticized Sen. Paul Laxalt (R-Nev.), a close friend of the president, and Sen. Jake Garn (R-Utah) for statements Thursday calling for rejection of the plan to shuttle 200 MX missiles among 4,600 underground shelters in their home states.

The senators argued that the MX should be put into existing Minuteman missile silos in other states, and they called for several other changes in strategic programs.

In a statement saying he was "disappointed" in Laxalt and Garn's action, Tower said they had advocated various alternatives "which offer the appearance of enhancing our strategic posture but which may actually fail to do so."

The Texas lawmaker urged his other Senate colleagues to refrain from committing themselves publicly to changes in the Air Force plan until a high-level commission of outside specialists chosen by the Pentagon and headed by Nobel Prize-winning physicist Dr. Charles Townes makes its report to the Defense Department and the president.

The Townes commission report is expected imminently.

Meanwhile, the military, political financial and strategic policy stakes on the MX decision are so high that lobbying has grown intense.

While most of the lobbying has been by opponents of the Air Force plan, Tower cost at least a temporary vote yesterday for the shelter-shuttle system, possibly combined with an antimissile defense system to provide additional protection for the MX shelters.

He said while the suggestions of Laxalt and Garn "may convey the illusion of real alternatives . . . I believe they constitute neither new nor effective substitutes" for MX deployed in the planned underground shelter system.

Specifically, Tower argued that the Laxalt-Garn plan did nothing to solve the question of vulnerability of fixed-based U.S. missiles to Soviet attack. tTower called the senators' suggestion that the administration also "reassess" the option to launch the U.S. missile force upon warning of attack "a dangerously destabilizing strategy, a policy we have no confidence we could implement."

Launch on warning means the president, upon being warned that enemy missiles were headed toward the United States, would give the signal to fire U.S. missiles before the enemy force hit, thus keeping the U.S. force from being wiped out.

This has always been an option, but every administration has stated that it is not a good one because it would give the president perhaps 15 minutes to make and execute an extraordinary decision.

Tower stopped short of accusing Laxalt and Garn of playing politics with the MX decision. But in a television interview he said that all considerations other than getting "the most survivable system that will give the best possible deterrence" against attack should be set aside.