On the day he yields the White House to Ronald Reagan, a subdued, thoughtful President Carter told reporters yesterday, he will fly home to Plains to begin "living the life of a former president," writing books and making no claim to be leader of the suddenly tattered Democratic Party.

A gentle Carter, in distinct contrast to the hard-edged, combative Carter of the campaign that ended in his overwhelming defeat nine days ago, stepped unannounced into the White House press room in mid-afternoon with no statement to make, no message to deliver, but simply to answer questions.

None of Carter's enormous success came easily to him. Even when he encountered defeat, it was not for want of a struggle. Yesterday, Carter seemed to have put struggle behind him.

He said he has no desire to run for president again, and when asked whether he might seek another term as governor of Georgia, he answered quickly: "No way."

Did he consider himself head of the Democratic Party? "No, not necessarily."

Carter spoke as though he were already looking back on his presidency and awaiting history's judgment of his achievements. In the future, he said, he will play "a fairly low-profile role."

"I have not quite so heavy a schedule these days as I have in the past," Carter said with a smile. He passed up many opportunities to criticize Reagan, and spoke only of supporting Reagan as best he can. The president did say he thinks his approach to relations with the Soviet Union is better than Reagan's, but that after leaving the White House "I will abandon my own approach and support his" because of the overriding importance of the issue of nuclear arms limitation.

Carter refused to be drawn into recrimination over what went wrong with his reelection effort. The president, who almost never makes jokes, answered only with one of the oldest in politics: "I obviously didn't get enough votes."

He invited the press to analyze what happened and history to judge him and his administration.

But in reviewing his administration, Carter gave history a hint how to reach its judgment: "One of the anomalies is that the things on which I worked hardest were the ones that were politically counterproductive," he said. The Panama Canal treaties, his Mideast policy and his human rights policy all hurt politically, he said.

In response to several questions, Carter permitted himself the luxury of answering "I don't know," something he almost never did when the cameras were on and his effort was to appear presidential.

"I don't know," he said when asked where the country will be in four years, given the conservative rise that swept Reagan into office and gave the GOP its first Senate majority since 1955.

"I don't have any particular candidate," he said when asked who should head the Democratic National Committee as it tries to rebuild the party.

Nor did he want to name a leader for the party. "Fritz [Vice President Mondale] and I actually are almost as close as brothers," Carter said. But, perhaps because he knows that the blessings of a repudiated president whose relations with the party mainsteam were never close is not a prize a brother might covet, Carter did not try to dub Mondale his heir.

Carter defended his record on balancing the competing pressures on a president to practice fiscal responsibility while also showing concern and compassion for those citizens who need help.

"In my own highly biased judgment, we have managed those two reasonably well," Carter said. He came closest to criticizing Reagan when he added that he hopes that the compassion is not lost in the nation's swing toward conservativism.

Carter said he would like to play a constructive role in the future by speaking out on issues that concern him. He mentioned the environment, consumers, minorities, human rights, jobs and halting the proliferation of nuclear weapons. "I reserve the right to speak out as my be appropriate," he said.

Carter stood behind the lectern his press secretary, Jody Powell, uses for daily White House briefings, and spoke quietly into the microphone, turning to look directly at each questioner.

Carter said he intends to write more than one book and that he will arrange for his presidential papers to be kept in a library in Georgia -- probably in Atlanta. Will there be surprises in his memoirs? "I hope so," Carter said.

He will also do some teaching and lecturing. "I intend to become a good fly fisherman," he said.

Carter said he might work for his church or for the Lions Club of Plains, but considers that former presidents should not get involved in business. Once he was quoted as saying he would like to be a missionary, but Carter said that was a misquotation.

The president was asked about the American hostages, who became so entwined in his last White House year and his final campaign. Did he have any timetable in mind for their release from Iran? he was asked.

Carter let the pain show a bit: "I've had a timetable in mind for more than a year now that never has been reached."

There was also the question about what advice he will give his successor when the two men meet next week. Carter thought for several seconds, and then said he would prefer to give his advice to Reagan privately.

The frustrations of the presidency clearly were on his mind. "I had a lot more advice coming in than I have going out," Carter said, remembering the briefcases full of ideas and enthusiasms he brought to the White House less than four years ago.