President Carter's weekend call for halting all nuclear testing "instantly and completely" has begun to bend to the realities of hard bargaining in the nuclear age.

The Carter administration declared its support yesterday for two pending U.S.-Soviet treaties that would permit the continuance of limited underground nuclear blasts. In the presidential campaign however, Carter criticized one of the treaties as "wholly inadequate," and called the other dangerous."

Yesterday, State Department spokesman Frederick, Z. Brown described the two disputed treaties as "way-station" for movement toward the ultimae objectives" held out by the President.

What the President said in his Sunday interview on seeking an early end to all nuclear testing, Brown said was an objective. What is before the Congress, Brown said, are two treaties "which are a step in the direction in which the President has clearly indicated he wants to go."

Critics are divided however on whether the two treaties are steps in the right direction or worse than no agreement at all. Carter appeared to share the latter viewpoint during the campaign.

Critics are divided, however, on whether the two treaties are steps in the right direction, or worse than no agreement at all. Carter appeared to share the latter viewpoint during the campaign.

A special study has now been ordered by the White House, through the National Security Council, to work out U.S. strategy on a total underground test ban, to be negotiated with the Soviet Union. The Carter administration has now decided to attempt simultaneously to get the Senate to ratify the two pending treaties. The new study on nuclear testing is the 16th special study now described as Presidential Review Memoranda, ordered by the Carter administration on major international issues.

The treaties, which have languished in the Senate for months, surrounded by controversy, limit underground nuclear weapons tests, and so-called peaceful explosions of nuclear devices for civilian purposes.

In 1963, a breakthrough agreement banned the testing of nuclear devices everywhere but underground. China, which continues to test in the atomsphere, is one of the non-signers of this international accord.

The pending U.S.-Soviet treaties are an attempt at further limitation. They would ban nuclear blasts bigger than 150 kilotons, or the equivalent of 150,000 tons of TNT. In order to distinguish between weapons tests and "peaceful" tests, the second of the new treaties permits inspectors to visit the site of the non-weapons blasts, under certain limitations. This procedure, which required two years to negotiate, was hailed by the Ford administration as a major breakthrough in Soviet secrecy.

Opponents in the American arms controls community made two basic criticisms, which Carter also raised in his election campaign. They criticized the 150-kiloton limit as too high, and charged that formalizing "peaceful nuclear explosions" was a step backward in the arms race, because nonnuclear nations under the guise of a peaceful program.

India entered the world's select nuclear club through the "peaceful explosion" route.

Carter said in the campaign:

The so-called Threshold (weapons) Test Ban Treaty represents a wholly inadequate step beyond the limited test ban of 13 years aog.

"The so-called 'on site' inspection provisisons of the Peaceful Nuclear Explosions (PNE) agreement signed recently may be a concession in Soviet eyes. but contrary to [Ford] administration claims they are no compensation for the PNE agreement's dangerous legitimizing of peaceful nuclear explosions, which are indistinguishable from bombs."

Nevertheless, the State Department said yesterday, "The (Carter) administration wants Congress to consider and approve the two test treaties currently before it as steps to an overall ban on nuclear tests."