President-elect Ronald Reagan has found an ally for his prudent, go-slow approach to a new strategic arms control treaty in an unlikely place: President Jimmy Carter's Oval Office.

When Carter late last year quietly put his signature on a classified, unpublished presidential directive known as PD50, he radically transformed his administration's pellmell approach to the SALT process. He also unwittingly handed his successor potent political ammunition by ordering that all future arms control proposals be "fully supportive of our national security."

Reagan has every intention of keeping arms talks with the Soviets in limbo until he has analyzed overall national security needs. That will not happen until his new national security team in the State and Defense departments and at the Arms Control and Disarmament Agency has completed "cleaning up the wreckage" of the last four years (as one Reagan aide told us). That means it will be months from now -- possibly late summer -- before Reagan is ready for anything resembling serious nuclear talks with Moscow. On arms control, Reagan is not a man in a hurry.

The president-elect and his top advisers want first things first. Before addressing complex arms control questions with the Russians, Reagan wants to lay out all defense requirements of the United States. That includes force levels for both weapons and manpower, the likely need for futuristic anti-satellite warfare, the possible necessity for a much-expanded nuclear testing program, exactly how the powerful new MX missile should be fitted into the nuclear arsenal and much more.

"There is no SALT negotiating strategy now and won't be for quite some time," one Reagan adviser told us. "Gov. Reagan is not about to move into that area in any haste."

The contradiction between Reagan's first-things-first approach and the unseemingly haste that galvanized Carter and his arms control lobby four years ago is both real and symbolic. Even before taking the oath of office, Carter was issuing orders to the Joint Chiefs of State for studies pointing to radical reductions for long-range nuclear missiles. SALT came first, and all else hung from it. With Reagan, overall defense stratgey comes first.

But Carter had undergone significant change by Aug. 14, when he approved PD50. That was two months after his SALT II treaty with the Russians ran into a hornet's nest of political opposition. The presidential directive was attacked and ridiculed privately by the arms control lobby still riding high in the Carter administration. But Carter stuck to it.

Written by a senior Soviet specialist on Zbigniew Brzezinski's National Security Council staff, the directive now turns out to be a remarkable fit for the arms control philosopy of President-elect Reagan (who regards SALT II as a dangerous piece of paper he would never have signed).

Carter's switch to caution was signaled in three basic questions spelled put in PD50: Does arms control contribute to achieving U.S. defense and force posture goals? Will it restrain U.S. adversaries and help U.S. allies? Will it truly limit the "likelihood of conflict"?

How Carter, the convinced arms-controller, became converted from all-out entusiast to healthy skeptic (despite campaign rhetoric attacking Reagan's alleged warmongerism) is hidden in the mists of contemporary U.S.-Soviet relations. One factor was the gradually emerging folly of the Carter State Department's pushing for Indian Ocean neutralization at the very time the Soviets were building immense new power along the contiguous shorelines of Asia and Africa. Another was the folly of negotiating a U.S.-Soviet limit on conventional arms aid to friendly foreign countries when the Soviets were pushing over $1 billion of arms on Ethiopia.

Whatever it was that changed Carter, his promulgation of P.D. 50 now becomes strong reinforcement for the long, cold look that Reagan is giving the SALT process and other arms control fancies. After studying P.D.50 recently, one senior Reagan adviser told us: "It's a good document. It helps explain why Reagan would never let arms control drive his foreign policy."