President Jimmy Carter, on the crest of high popularity and loving his job, momentarily regressed at his May 26 press conference to the bad old days when, as "Jimmy Who?" he was an insecure long shot for the presidency.

Pressed to defend his Korean troop-withdrawal policy a few days after the Singlaub incident, the President reached too far for expert witnesses. "President Park himself, the president of the Republic of Korea, has called for the removal completely of American troops," Carter said. Whether he knew it or not, that simply is not true, or even close to true.

Later came a far milder recurrence of the old hyperbole syndrome. Pressed to defend his conduct of strategic arms limitation talks, he defended their openness. In point of fact, since the breakdown of the Moscow talks, the subject has been secret enough to make even Henry Kissinger envious.

Compared with the whoppers committed in obvious self-service by Lyndon Johnson and Richard Nixon, these might seem trivial stretche sof the truth - the Korean statement a minor fib, the SALT comment a mild exaggeration. Yet, for a President whose popularity depends on public perception of honesty more loudly proclaimed than by any predecessor, any deviation from literal truth by Carter is potentially serious.

Moreover, the May 26 statements were distressingly reminiscent of his early presidential campaigning. Once he became the front runner in 1976, Carter generally treated reality with more care and has continued to do so as President.

Accordingly, the statement about President Park, while not of overriding importance in itself, poses a question: Will this become a Carter characteristic when the inevitable days of hard crisis set in?

When we sought the source of the President's claims about Park, the White House press office referred us to the National Security Council, which suggested the State Department. A State Department official sent us back to the White House with the comment: "When the President messes up, it's the White House's job to get him out of it."

Nevertheless, one official at State did come up gamely with an explanation: that Park had set as the South Korean objective a strengthening of its own forces to make it capable of defending against a North Korean attack without the help of U.S. troops.

That scarcely advocates removal of U.S. troops. Indeed, the South Korean president has vigorously opposed removal. "I would guess," a White House aide told us, "it would be more accurate to say Park 'reluctantly acquiesced' rather than 'advocated.'"

The President's questionable statement on SALT was more understandable. Challenged as to whether his public discussion of U.S. policies was hurting this country's negotiating position on SALT and the Mideast, Carter said: "I think it's good for the American people to know what our positions are at the time tha tthe Soviets know what our positions are, and vice versa. This is a matter that must be addressed openly."

But the new U.S. arms-control package is not being openly discussed, as was the earlier package rejected by Moscow. Not only do the American people not know what is int it, but moreover it has been tightly held at top levels of the State Department and disarmament agency. Even the joint chiefs of staff were uninformed until just before it was presented to the Russians in Geneva.

When 10 key senators were summoned to the White House to talk about SALT before the Geneva meeting, Carter would divulge no details. The U.S. proposals came out a week after the Russians heard about them - and then to a closed Senate subcommittee session with the senators sworn to secrecy. To this moment, there has been no official public disclosure of the current proposals to limit nuclear weapons.

Considering Oval Office perfidy in recent years, Carter's liberties with the whole truth appear trivial. But the question remains whether they portend how Jimmy Carter will react when days of adversity begin to afflict his presidency, as they surely will.