Time is running out on President Reagan in his effort to achieve an arms-control agreement based on Soviet fears of the U.S. military buildup and the Strategic Defense Initiative.

With the SALT II treaty under a death sentence and prospects for a 1986 summit slipping, Secretary of State George P. Shultz has become the good-soldier advocate of policies he does not support and did not advocate.

White House insiders say that, before Reagan announced his intention to abandon SALT II, Shultz wanted a simple statement noting that the United States would exceed the treaty limits later this year when additional B52 bombers armed with air-launched cruise missiles are deployed. Instead, Reagan adopted the formulation favored by Defense Secretary Caspar W. Weinberger and renounced the treaty.

What concerns arms-control advocates within the administration is less the weak reed of SALT II, which has been violated by the Soviets, than the vacuum in national-security planning in the White House. National security affairs adviser John M. Poindexter has neither interest nor experience in arms control and has left Reagan alone and vulnerable at a critical time.

Poindexter, a Navy vice admiral, sees his role as the dutiful military one of forwarding conflicting views of the Pentagon, State Department and other agencies to the president without putting his stamp on them or worrying if they make sense. This has simultaneously earned him a reputation as an honest broker and an ineffectual technocrat who lacks insight into Reagan.

A Republican close to Shultz complained recently about a "lack of vision" on arms control at the White House. Actually, there are competing visions. Neither Weinberger nor his arms-control specialist, Richard N. Perle, has ever wanted an agreement with Moscow. But Shultz and diplomat Paul H. Nitze believe that Soviet economic and political needs create the possibility of agreements that could keep the arms race within bounds.

Unfortunately for the coherence of his presidency, Reagan shares both views. His never-trust-the-Soviets instinct echoes Weinberger while his desire to depart the presidency as a peacemaker is responsive to Shultz.

Poindexter's three predecessors in the administration differed in ideology, experience and temperament but understood that Reagan's approach to the Soviets is full of conflicts. All tried, with varying success, to hammer the internal differences into a rational consensus.

Reagan tilted toward negotiation in 1985, when national security affairs adviser Robert C. McFarlane took advantage of the climate engendered by the forthcoming feel-good summit in Geneva to isolate Weinberger and impose rare discipline on the administration's arms-control approach to the Soviets.

With a second summit in doubt, hard-liners have seized their chance by persuading Reagan to denounce SALT II without offering anything specific in its place. But this week a coalition of centrist arms-control advocates plans to try to fill the Poindexter vacuum by announcing the start of a study to recommend an arms-control policy to succeed SALT II. The group includes includes key senators of both parties and is chaired by McFarlane; Harold Brown, defense secretary in the Carter administration, and Brent Scowcroft, national security adviser to President Gerald R. Ford.

No matter what comes of this, Reagan is unlikely to move toward arms control unless someone weighs in at the White House. The best candidate for such a useful task, albeit a long shot, could be White House chief of staff Donald T. Regan, who has yet to distinguish himself in foreign affairs.

Regan, secure in his operational control of the White House, recently has become unusually interventionist on controversial issues. He raised important questions about the space shuttle and pressed Poindexter to provide answers. He interjected a cautionary note into the pell-mell pursuit of leaks. He has the president's ear and has shown on military budget issues that he is unafraid to stand up to Weinberger.

Regan is no peacenik, but he understands that the repudiation of SALT II has damaged congressional prospects for the president's national-security policies. If the president acts on Regan's insight, Reagan might still be persuaded to give arms control a chance.

Reaganism of the Week: Advising senators last Friday on how to handle televised coverage of the Senate, the president said: "Learn your lines, don't bump into the furniture and, in kissing, keep your mouth closed."