President Carter has stamped out the most damaging opposition within his own official family to sale of AWACS radar planes to Iran, but his decision to force the issue in Congress is still one of his most courageous and most dangerous steps since taking office.

Before the August recess, unexpectedly severe opposition forced Carter temporarily to pull back from Congress the proposed $1.2 billion transaction. Liberals opposed to arms sales in general and to authoritarian Iran in particular hoped he would not renew the transaction in September. But the President is going ahead, both to provide early warining against surprise Soviet attack and possibly to stimulate AWACS purchases by Western European states.

Even broader questions are involved. If the sale is killed by both houses' voting against it, a landmark will have been reached in the executive's steady loss of foreign-policy power to the legislature. That encroachment has been in full swing since the trauma of Vietnam.

These congressional foes have been helped by allies within the Carter administration. What particularly damaged Carter in July was the stunning attack on the plan by his close adviser. Stansfield Turner, director of the Central Intelligence Agency.

Turner's opposition, never even discussed with the President, was based on concerns of espionage: Soviet agents might capture a super-secret encipherment gear used in the aircraft. Actually, that device was never intended to be used in the planes destined for Iran.

So, in a confidential letter to the President last week, Turner said "additional measures" taken by the United States and by Iran to minimize the security risk "should materially reduce the likelihood that physical compromise will in fact take place."

Similarly, muted opposition by Jessica Tuchman, a National Security Council aide on arms-control matters, has ended. Tuchman gave ambiguous answers to one uncommitted congressman who called her in July for guidance on the deal. Now she is convinced the sale is in the U.S. interest.

But elsewhere in the administration, Carter's policy is being subtly undermined - particularly by the Arms Control and Disarmament Agency. This is reflected in ACDA's sabotage of another proposed weapons deal. Administration officials are convinced that ACDA's opposition to the sale of F-15 fighter aircraft to Saudi Arabia was leaked to The Post by Thomas Hirschfeld, ACDA's deputy director of weapons evaluation.

Sale of the sophisticated fighter to Saudi Arabia seemed to its ACDA critics to violate the President's campaign edict against introducing advanced weapons systems to an area that did not have them. That is precisely what motivates many liberals on Capitol Hill in fighting the AWACS sale to Iran.

Another undecover critic of the plan is Dr. Lynn Davis, Deputy Assistant Secretary of Defense for policy and plans and principal author of the notorious PRM-10 study on U.S. forces. She felt so strongly that she wrote a letter outlining her objections to Secretary of Defense Harold Brown, according to one well-informed congressional aide. But Brown's own support has mousetrapped Davis. Brown informed the President in a private memorandum last week that "it is my personal view that there is always some risk" in any arms sale, but this one has "clearly adequate safequards."

That any President should have to work so hard to ensure administration support for his policy tells much about the unique problems Jimmy Carter faces in adjusting his promises - and some of his national-security appointments - to fit the ugly facts of real life.

A principal objection privately given the President by fence-sitting congressmen involves that very question: The AWACS sale to Iran makes a mockery of Carter's public pledges, and hence us politically dangerous to him. Rep. Lee Hamilton of Indiana, a key member of the House International Relations Committee, told Carter in the Oval Office last week that the Iran deal could undermine his "integrity."

The battle will be decided in the House; the Senate is stacked against the sale. Sen. Hubert Humphrey's terminal cancer dealt a shattering blow to Carter's hopes of getting his plan approved by the Senate. With Humphrey absent, his post as chairman of the foreign relations subcommittee handling arms sales goes to Sen. Frank Church. Humphrey leaned toward the sale; Church was one of a score of senators who wrote a blatantly critical letter about it to Carter.

If the President loses this battle, he may well have lost the whole arms-sale war. That would further whet the congressional appetite for ever more control over the details of U.S. foreign policy, with potentially grave consequences for the nation.