Jimmy Carter has a terrible secret: Deep down he is softie. This has been missed by most of the political pros who have been exposed to his no-nonsense manner. One even characterized him as "tough as a warehouse rat." The old pro meant it as a compliment, believing a president should be hard.

But those who know the real Jimmy Carter have detected that, behind the glacial stare, the twitch in his jaw and the crisp memos that betray his angry moods, there, beats a heart of mush. He's too trusting; he lacks ruthlessness; he'd rather be Mr. Nice Guy.

Carter can see and smell the Atlanta ghetto where he once worked quietly as a lay Baptist, intimates say. He can feel the impoverished Indian village where his mother labored as a Peace Corps volunteer. He identifies with those who need succor.

When Assistant Treasury Secretary Laurence Woodworth was seized with a severe stroke, a concerned Carter placed a private telephone call to the doctor. Afterward, the president interupted a Cabinet meeting to talk about the stricken subordinate. Woodward "is one of the finest, most competent, most dedicated public servants this country has ever had," said Carter with quiet emotion.

When Ralph Nader assailed his former colleague Joan Claybrook in her new capacity as the federal highway-safety czar, she received a private, reassuring telephone call fromthe president. He told her she was doing "a good job" and "not to worry."

Sources close to the Carters confide that it takes Rosalynn to raise suspisions about an associate. I discussed this with him once in the Oval Office. "I think I trust people much more than she does," he said. Then he paused and corrected himself. "Some more, maybe not much more."

But then he went on: "Yes, I have a feeling I can trust people. And I have had a lot of experience with a lot of different people. And I have been very, very rarely disappointed or betrayed."

He has steadfastly refused to feel betrayed, for example, by Bert Lance. At the height of the public revelations about Lance's financial shenanigans, the president told me: "I know Bert Lance . . . I know what he has done on the past as far as serving the people that trusted him and as far as his competence is concerned.

"He was the first person that I thought about when I was finally sure that I would be elected president, and I wanted him in a major department that had profound influence on the rest of the government and still work directly with me, someone I could trust. And I chose Bert for that job, and I have no reason to think that I made the improper choice."

Another who has gained Carter's confidence, special trade representative Robert Strauss, has found him to be a soft touch. The president issued orders that all government officials should fly tourist class. Strauss ignored the policy and continued to fly first class. White House budget officers notified him that he would have to sit in the back on airliners. Strauss retorted that he had always flown first class and would continue to do so. His refusal was reported to higherups. Word came from the Oval Office that Strauss could fly at whichever end of the airplane he chose.

We have had access to confidential Cabinet minutes, and have not found a harsh word that Carter might have spoken to his subordinates. Not until he summoned them to Camp David on the April 15-17 weekend for the painful review of past mistakes was he the least unpleasant. A few excerpts:

Nov. 21 - The president "is pleased with the harmony that prevails among the Cabinet; between the Cabinet and White House staff."

Dec. 5 - "The president commended the vice president and White House staff for an outstanding job of preparing analyses and proposals for next year's presidential agenda."

Jan. 30 - Carter boasted that "he has generated a compatability among the strategic planners of our government unknown in former administrations. He commended [Defense Secretary Harold] Brown for his leadership."

Feb. 6 - "The president commended [Agriculture Secretary Bob] Bergland who, he noted, has been under the most intense pressure for months, for his wise and calm handling of the farm strike situation."

Feb. 13 - "The president welcomed [Undersecretary of State Philip] Habib, who has recently recovered from a heart attack and called him 'one of the great men of our country.'"

March 6 - "The president concluded by saying that our economy is basically sound and our country is strong." He described the administration's relations with Congress as excellent and said that he has faith in the "tremendous unshakable strength of our people."

It wasn't strictly true, of course, that harmony prevailed among Carter's subordinates, or that the Cabinet has done a superb job, or that the administration's relations with Congress are excellent. Last month at Camp David, Carter finally faced the facts. He asked his personal adviser, Atlanta attorney Charles Kirbo, to gather the Cabinet's grievances against the White House complaints against the Cabinet.

Carter confronted his subordinates with the rival charges and ticked off his own catalogue of complaints. "I've looked at the public opinion polls," he said grimly, "and my job rating is about 50 percent." He told them, in effect, to shape up or ship out. It was the sharpest language they had heard him use.

The wind from Camp David has brought a tightening and toughening. But it is against Jimmy Carter's nature to be mean. I once asked him: "Do you feel that a president has to be ruthless sometimes - in other words, put the welfare of the country ahead of the welfare of his friends?"

"I think so,"said Carter.

But he didn't sound convincing.