While publicly taking a tough election-campaign stance on defense, President Carter has privately ordered a delay in one of the most sensitive areas of national policy: an overdue increase in the production of nuclear explosives for warheads to power America's expanding arsenal.

In response to Ronald Reagan's tough rhetoric, the president has given maximum publicity to the new "Stealth" aircraft and the new nuclear targeting policy to show progress against Soviet power. The delay over expanding nuclear fuel production suggests procrastination in hopes that the Soviet threat will somehow ease. That raises questions about how much the Carter team's view of the Kremlin really has changed.

This other face of the administration's defense policy also points to an election-year political dilemma. The delay in nuclear explosives production betrays a fear that expanded production would antagonize the anti-nuclear and environmental lobbies. But the delay itself generates fear about the reaction of defense-oriented congressmen.

That second fear was reflected in a "top secret" directive on Aug. 7 signed by Zbigniew Brzezinski, Carter's national security adviser. The tip-off was this instruction: "All responses to press inquires about special nuclear materials should be low-key and . . . limited."

Brzezinski's memorandum went to the secretaries of state and defense, the CIA director and the Joint Chiefs of Staff. It informed them that "a formal . . . decision" to expand nuclear fuel production will be "deferred" until an international conference on nuclear proliferation in Geneva has ended. That will not be for weeks.

The decision to delay means the detentists have won the backstage debate between them and the defense community, a debate waged in Congress and within the administration. Their victory means in essence that there is a high probability of running out of nuclear explosives for the new weapons systems. The administration is authorizing rifles but not the bullets they can fire.

The "bullets" necessary were spelled out last spring in a classified document written by the Joint Chiefs of Staff for Secretary of Defense Harold Brown, setting production targets for new weapons-grade materials. These targets were required by Carter's recent decisions to build the MX mobile missile and the Trident II submarine-launched missile and -- most important -- to put warheads on new nuclear weapons for Europe to compete with existing Soviet systems.

"Carter has approved these news systems with much fanfare," one defense expert told us, "but the delay in new production of weapons-grade fuel does not give the Joint Chiefs a Chinaman's chance in hell to put warheads on them."

In its report on the authorization bill for the production of warhead fuels last May, the House Armed Services Committee said that Carter was planning major new nuclear weapons for which there would be no explosives. "Is there a commitment to a nuclear weapons program?" the committee asked.

Pentagon officials have tried to argue this case in congressional testimony short of publicly breaking with the White House. On June 18, James P. Wade, Brown's assistant for atomic energy, testified cautiously that "additional resources" are needed.

Congress has been unsuccessfully pressing Carter for several years to gear nuclear plants in Hanford and Richland, Wash., for production of modern, weapons-grade nuclear fuel. But when, after months of procrastination, Carter finally took the lead in modernizing NATO nuclear weapons and ordered full-speed on the MX and Trident missiles, defense-oriented congressmen believed the impasse was broken.

Brzezinski's directive makes clear they were wrong. It revealed that a July 24 session of the Presidential Review Committee formally decided to defer new production facilities.

The directive went on to warn that "responses to press inquiries . . . should be low-key and limited to the following points": present production is "adequate for the near term"; "long-termrequirements are under review but are difficult to predict"; if higher production is needed, "appropriate programs will be requested."

In yet another gesture to politics, the directive orders that all responses to congressional inquiries "should be cleared" by the Defense and Energy departments, the NSC and the Office of Management and Budget. Having put the development of "Stealth" on the network news, Jimmy Carter wants to keep his other defense face shrouded from public view.