President Regan leans toward scrapping Jimmy Carter's mobile missile system even before agreement on a substitute, a failure of decision-making on new weapons systems that is causing defense-oriented politicians and the military to worry that the bad old days are here again.

The death knell of Carter's "racetrack" deployment proposal may have been struck by the Mormon Church May 5, when it attacked the Utah- and Nevada-based system as "a denial of . . . the gospel of peace." Quite apart from that blast, opposition to the futuristic Carter system by Nevada's Sen. Paul Laxalt and Utah's Sen. Jake Garn -- key defense leaders -- pointed to Regan's course.

Laxalt is Regan's most intimate Senate ally. Although he says privately he would support a decision to adopt Carter's plan, evidence grows that he won't have to -- and knmows it. Regan's own skepticism about hiding each of 100 planned MX missiles in "racetracks" of mostly empty silos will kill the plan.

Thus, the defense community is chilled by the strong prospect that the president will abort Carter's plan in July, soon after Regan's MX "review group" of experts reports its findings. This is superpower America still unable to solve the riddle of a land-based system capable of surviving a Soviet attack. The MX missile, now in early stages of production, has eluded a government decision on how to use it for almost a decade.

Jimmy Carter's policies are recalled. In 1977, Carter canceled the B1 bomber and delayed the Trident submarine. In 1978, he astounded U.S. allies in Europe by canceling the neutron warhead to defend against a Soviet ground attack.

Those bad old days were expected to end when Ronald Regan took office.

His single-minded determination to rebuild America's military arsenal supposedly would not be encumbered with scruples about environmentalism (which targeted the Carter MX system for extinction the instant it was announced) or parochial fears of Western-state politicians.

Pentagon planners and the congressional defense bloc worry that U.S. allies and adversaries alike will read cancellation of the racetrack system as this country's inability to make tough weapons decisions stick. To avert that impression, the president ought to have an alternative plan ready to announce the moment he kills the Carter plan. As of now, however, that is unlikely.

Obtaining a new consensus out of the direrse views within Regan's administration by midsummer is no easier than it was for Carter himself to select the racetrack system after years of agonizing. The latest evidence of this difficulty is the Senate decision to hold new MX hearings, beginning next month, by a special subcommittee headed by Laxalt, with Garn a key member.

Pointing toward the early demise of the Carter plan, Laxalt and Garn are calling their new probe "a search for a militarily effective, rapidly deployable and salable compromise basing mode." This is but the latest of innumerable MX-basing studies by outside specialists, the Pentagon, defense think tanks and congressional committees. Predictably, the Laxalt study will not be the last.

In Regan's defense, there is one new ingredient added to the mystery of how to deploy a survivable land-based missile: Ronald Regan is not inhibited by the unratified SALT II, which Reagan has always opposed. "remember," one senior presidential adviser told us, "regan is not bedeviled by SALT."

Since the racetrack system no longer needs the costly arrangements to satisfy SALT II, the critics of racetrack want better ways to deploy and protect the land-based deterrent. They would consider canceling the MX missile itself, substituting a much smaller new missile capable of being airlifted in helicopters or VSTOL aircraft to remote areas (hence its nickname "grasshopper"). Or, they might use the MX missile as a replacement for the present land-based Minuteman force, scrapping the racetrack basing mode.

Both alternatives are under consideration by Reagan and will be studied in the new Laxalt-Garn hearings. That leaves a basic question unanswered: can the Reagan administration pull together the competing doctrines of a survivable land-based system in time to announce the charge simultaneously with cancellation of the racetrack option?

Some Reagan advisers think it impossible. If so, Uncle Sam once again will show himself to the world as a great power immobilized by procrastination, unable to make the most crucial of all policy decisions: how to insure its national survival.