The suprising delay in test-firing a new 40-kiloton warhead designed for a Poseidon missle-firing submarine is just one striking illustration that the Soviet move into the oil-rich Persian Gulf area has not yet weakened anti-nuclear forces ensconced in the Carter administration.

Equally worrisome is the tenacious holding action by powerful environmentalists against congressional demands for faster production of weapons-grade nuclear materials to carry out Jimmy Carter's pledge to "modernize" NATO's nuclear weapons.

The unpublicized delay of the relatively low-yield Poseidon warhead test has now lasted more than five weeks. During that time, the warhead that is designed to "rehabilitate" part of the submarine leg of the U.S. nuclear triad -- land, air and water -- has been resting quitely in its underground cradle at the Nevada test grounds. But the president has not yet signed the order that he alone can sign giving permission for the underground test.

At stake here is nothing approaching American contemplation actually to fire in anger the Poseidon or any other ICBM. Far from it. At stake is whether the U.S. deterrent, particularly now that SALT II has been at least temporarily shelved by Soviet aggression, is to be kept in top working order.

The president is being pressured by his own environmentalists and other powerful anti-nuclear forces, including penny-pinching bookkeepers in his Office of Management and Budget, to go slow on testing. But the Soviet Union is in the midst of highly productive underground testing that proceeds at a rate at least twice the U.S. rate. So long as detente was enthroned and Washington and Moscow at least appeared in agreement over future limits on strategic arms, this was politically salable.

Today, however, it is not. Long knives on Capitol Hill are being sharpened to white down the power of the Council on Enviromental Quality of the president's Office of Science and Technology Policy and of lesser administration officials, some on the National Security Council staff. These officials were hired long before Carter's eyes were opened By his own admission) by the Soviet invasion of Afghanistan.

Thus, in a little-noted, year-end report, a House-Senate conference committee warned that unless the production of weapons-grade nuclear fuel is drastically increased, the United States will be unable to carry out its commitments to modernize forces in Western Europe or to build warheads for the new MX missile in this country.

The conferees on the nuclear weapons program authorization bill were blunt: "[we] are concerned that billions of dollars may be spent to procure delivery systems which cannot be armed because sufficient funds have not been committed to testing or because critical decisions have not been made to produce sufficient quantities of special nuclear materials."

Their demand: that Carter immediately reactivate a Hanford, Wash., reprocessing plant known as PUREX. Working with nuclear power reactor next door that would supply plutonium, PUREX would manufacture fuel for the new nuclear weapons.

But the latest version of a draft presidential report now circulating inside the administration makes it all but certain that Carter is being talked out of reactivating the PUREX plant. Why? Because, the president is being told, to do so would contradict three years of his preachments against reprocessing plants that might proliferate weapons-grade materials for building nuclear bombs.

The congressional conferees who signed the compromise report calling for reopening PUREX are not weak-kneed innocents. They include the chairman of the Armed Services committees, Sen. John Stennis and Rep. Mel Price, and such powerhouses as Washington's Sen. Henry Jackson, long a protecter of Hanford's essential nuclear role, and Sens. John Tower and Strom Thurmond.

What makes the president's resistance to the clear mandate of Congress doubly troubling is that Secretary of Defense Harold Brown specifically testified on Capitol Hill Dec. 18 that he had "recommended to OMB that PUREX be restated and funded" for the 1981 fiscal year. The cautious Brown made it clear that he worries about adequacy of special nuclear weapons materials both for testing and for arming the new array of nuclear weapons needed to keep the Untied States at or near par with the Soviet Union.

Unless the president changes his mind and downgrades his enviromental experts, he will soon be defending himself against serious charges that, despite his admission, the Soviet ivasion of Afghanistan did not open his eyes wide enough to the new reality.