On the morning of the 24th day of siege in Iran, with a crucial nighttime news conference ahead, President Carter was short of temper.

"Where's Jody and Ham?" the president asked, looking around the Oval Office at 9:45 a.m., at the start of the regular meeting of the highest level staff that is camouflaged on each day's public schedule as a private meeting with "congressional liaison chief Frank Moore."

"They'll be here in a few minutes," said Alonzo McDonald, Hamilton Jordan's enforcer.

Carter glared. From the tight knit of his brow, his advisers could tell he was angry. Also from what he said.

"Tell them if they can't be here on time, don't come at all," Carter said, according to a source who was there.

Glints of anger, cold and metallic (never hot or explosive), have come at various times from the president as he has grappled with this crisis that looms as perhaps the most significant event of his presidency. t

At that moment Wednesday, Carter was concerned about, perhaps even preoccupied with, the fact that he was going to hold a news conference, despite the cautious contrary advice of his aides in the midst of his greatest crisis.

(He had forgotten, according to one source, that Hamilton Jordan and Jody Powell were, at his instruction, at the daily meeting of the National Security Council's special coordinating committee that oversees all policy on the Iranian situation.)

The news conference proved to be Carter's best as president. His answers were firm but measured, and he spoke with uncharacteristic force, down to the warning of "grave consequences . . . if harm comes to any of the hostages."

Carter appeared, under this pressure, more like a leader than he has at any time in his presidency. And this gets to a central political consequence of this grave international crisis:

The Ayatollah Ruhollah Khomeini may have succeeded in doing for Carter what he never could have done for himself -- and what all the rafshoonery at his command could not have done for him.

The crisis in Iran may well be giving Carter the credentials of leadership he has been lacking -- leadership that has been the flag around which Sen. Edward M. Kennedy has rallied his presidential challenge.

Carter could go out and make 49 more speeches about the virtues of energy mobilizations boards and energy security corporations and succeed only in making himself a moral equivalent of Sominex. He could not have changed his leadership image on his own. But now, with Allah's will as interpreted by the ayatollah, commander-in-chief of mob voilence, Carter may be doing just that.

Carter's overall job rating has jumped six percentage points since mid-October, according to an AP-NBC poll conducted last Tuesday and Wednesday. And 67 percent said they approved of Carter's handling of the Iran crisis, with 22 percent disapproving.

Carter's overall job performance was rated excellent or good by 30 percent (it was 24 percent last month), with 49 percent rating him only fair (about the same as last month) and 18 percent rating him poor (24 percent last month).

Another survey, taken Nov. 23 - 27 by a well-respected firm that has polled for Republican candidates, showed almost identical results.

"I think that Carter has turned the corner and is beginning to build momentum," this pollster said. He attributed the turn-around to improvement in perceptions of Carter's leadership, brought about by the Iranian crisis.

But the crisis has also exacted a powerful toll from the nation in general, on the hostages and their families in particular and on the president physically.

Jimmy Carter has visibly aged.

In the rear of the East Room, some of the president's aides are clustered to watch their boss at this new conference. The president enters from the rear of the room and walks to the podium in front. Bathed in the harsh kleig lights, he turns around.

"When he turned toward the audience I just kind of jumped," recalls one aide. "I mean it was a shock. I see him often, but I just haven't noticed it day to day. But that night, he looked very old."

In some ways, Carter has long appeared older than his years. His posture has long been slope-shouldered. Even during the early days of his 1976 presidential campaign, he was a 51-year-old man with that trademark Jimmy Carter grin, but with Miss Lillian's neck, heavily lined and with saggin skin.

One reason Carter looks so old may be that he has lost so much weight recently. He was down to 151 pounds fully clothed last Labor Day, and his 5-foot-9 frame now appears much slighter than that.

Such diverse intimates as his wife, Rosalynn, and his television imagist. Gerald Rafshoon, have been urging him to gain weight, sources close to Carter say. Much of his weight loss may be result of his heavy daily regimen of jogging, to which the president has applied himself with evangelistic zeal, running at least three miles a day. Says one adviser, "He could stand to gain 15 pounds. It would fill out his suits, not to mention his face."

But there is another factor that appears to have taken its toll in the etchings of the president's physique. It is the frustration of a crisis that has placed upon him that heavy burden of responsiblity for the lives of Americans whose fate is largely out of his hands.

It is a frustration that has surfaced from time to time as the president has dealt with the crisis in Iran.

There was that moment of anger in the early days of the siege.

In the morning meeting with his senior advisers, the president was uncharacteristically angry. He had given an order the day before that he wanted no demonstrations by Iranian students on federal property. Yet throughout the day he had dealt with a number of officials on this question and still, in the District of Columbia, there were some scattered demonstrations.

"I've never chewed ass over mistakes -- we all make mistakes," Carter said angrily, according to one source who was in that Oval Office meeting. "But when I give an order, I don't want to have to spend 12 hours on it the rest of the day."

After a month of frustrating and at times anguishing decision-making, the president and his advisers feel the situation may break. They expect the ayatollah will win overwhelming public aproval tomorrow of the constitution that will name him ruler for life. And that, they feel, is what he has been cold-bloodedly marshaling nationalist support (via anti-Americanism) to obtain.

No one knows whether the hostages will be freed, harmed or perhaps be tried and convicted of spying and then be expelled. But one senior Carter official said, "If something is going to happen, it will probably begin to happen this week." And the polls so far suggest that the public is prepared to give strong backing to whatever action Carter takes in response.

Jody Powell was musing the other day about the effect of the siege in Iran on politics in America. He recalled an observation by one historian that administrations are shaped more by what happens to them than by what they set out to do. For Powell, it was a recollection worth savoring, because the observation came from Arthur Schlesinger Jr., adviser to the late President Kennedy and supporter of the current challenger Kennedy. What Schlesinger actually said, writing of Kennedy in his book "A Thousand Days," was this: "Some of his greatest predecessors, he would sometimes day, were given credit for doing things when they could do nothing else."

EPILOGUE: Last Monday, Carter received a detailed written report, pulling together from various sources information on the treatment and condition of the hostages. Carter had been at his angriest, aides say, after Rep. George Hansen (R-Idaho) came our of the U.S. Embassy in Tehran and began issuing statements that the hostages were in generally good condition, even though he had been prevented from seeing many of them.

Carter had commented frequently on how awful every hour of every day must be for the hostages, and this detailed report painted a far grimmer picture than any of Hansen's mouthings. Carter wrote a note on the report -- "The world needs to know this" -- and sent it to Jody Powell.

But before Powell could read it, Carter already had walked over to the East Room to say a few words to some New England civic leaders. And although it was not planned for him to do so, Cater angrily, emotionally, gave the most detailed public statement yet on the situation at the embassy.

"They've been threatened, at times, at pistol point, and encouraged to make statement contrary to their own inclinations. They've not been given a chance to go outside to get any sun shine, to get any exercise. They've not been permitted to take baths or change clothes and this is a reprehensible thing -- a disgrace to every person who believes in civilization or decency."