After a long period of unaccustomed reticence, former president Jimmy Carter is beginning to speak bluntly about his defeat and about those he holds responsible, and to carve out a new role for himself in the national dialogue.

Part of his new life will involve increasingly explicit criticism of his successor, starting now. "I believe I've given President Reagan sufficient time to let his policies take shape" without Plains-spoken criticism, the Georgian said in an interview en route home last week from a 15-day journey to China and Japan, his first overseas trip since leaving the White House.

Carter acknowledged a continuing interest in defending and advancing the policies he pursued for four years in the Oval Office, but he forswore any personal interest in a political future. "Even in my private, secret thoughts and my private talks with Rosalynn" there is no prospect of a return run for the presidency, Carter said.

Advising a longtime associate to seek an independent career, he said recently that "I've lost the presidential itch and I don't expect it to return." Carter said in the interview, "I know what it took. I know the deep-seated ambition to win that I had. There's nothing like that" in his mind or heart today.

Nearly eight months out of office, Carter seems more at ease, less driven, and on occasion more reflective. In Asia he was able to joke about losing the election--for example, telling China's Vice Chairman Deng Xiaoping before cameras and reporters that he would have won if Deng had been his running mate. And when the new Communist Party chairman, Hu Yaobang, emphasized in a private meeting the importance of placing younger men in top leadership posts, Carter broke in to exclaim, "That's what I tried to tell the American people last year, but they went the other way!"

Carter told a reporter that he was never depressed by his defeat, "not yet," and did not interpret it as a repudiation of him or his policies by the American people.

"I think everybody in Washington, in disappointments and defeats, tries not to blame them on themselves, and maybe that's what I did," said Carter. For him, 1980 was "the most difficult year of my life...the whole year was kind of a quandary." Those who made it so, in Carter's view, were two K's: Khomeini and Kennedy. He is acerbic, close to bitter, about both.

"I'll always believe that had the Iranian revolution not come along--a big if--had we not had the doubling of oil prices with tremendous inflation plus the hostage seizure, I would have won" reelection, he said. In a Japanese television interview, Carter called Ayatollah Ruhollah Khomeini "unpredictable, weak in a time of crisis" and a man who "betrayed the basic elements of Islam's religion in perpetrating murder, encouraging kidnaping and terrorism."

As for Sen. Edward M. Kennedy (D-Mass.), the former president said in the Post interview that he was "really peeved" at being forced to spend so much of his political time and energy seeking support from the natural Democratic constituencies turned off by Kennedy's "vitriolic attacks." While Carter "tried to move the Democratic Party toward a more responsible fiscal policy and still retain the commitment to the traditional ideals," Kennedy "was going around talking about old people having to eat cat food because of my programs" and saying "I was callous about mentally retarded children."

"The Americans for Democratic Action and Kennedy and so forth didn't like fiscal responsibility, or anybody talking about a balanced budget, eliminating waste or phasing out a program, or putting in efficiency." Carter said they considered this attitude "a betrayal," while he believed it was realistic and essential.

The ex-president drew a distinction between his pruning of domestic programs and the Reagan administration's large-scale cutbacks. Disputing the commentators' often-expressed belief that Reagan is creating a new American political alignment, Carter said, "I don't detect any reasons why a responsible Democrat who took my own policies could not prevail against anything that Reagan stands for. My guess is that Reagan's record will be one of vulnerability."

Among the vulnerable points, as Carter foresees them:

"A tremendous trend toward shifting financial responsibility, and therefore heavier taxation, toward state and local governments. That means away from the wealthy who finance the federal income tax system onto the people who pay property taxes and sales taxes, and in the process the service to people is being cut." For all these reasons, Carter said, Reagan is going to find it difficult to win the support of mayors and local officials.

Unemployment, which is "going to be a problem," as demonstrated by recent increases even before federal job programs are phased out.

Holding the North Atlantic Treaty Organization together in the face of "the inclination the administration has to ascribe all of the world's problems and conflicts to the Soviets, and to make them kind of a superman, a bugbear." The allies don't agree, he said.

The fiscal problems inherent in doubling defense expenditures in five years, increases well beyond those proposed by Carter. He is specifically critical of the B1 bomber, which he calls "stupid, and a gross waste of money;" production of the neutron bomb "when there is no one willing to deploy it" in Europe, and the airborne MX missile, "a silly thing to even talk about seriously" because of its vulnerability.

Policies toward human rights, the Third World and other areas emphasized by Carter and downgraded by Reagan.

"The point is, not to be critical of President Reagan and his administration, that there are some difficulties built in that haven't changed--the same problems we're struggling with now are the ones I struggled with for four years," the former president said. At another point, Carter mused, "if the answers were easy, they'd have been found long ago."

Political differences aside, Carter's attitude toward Reagan may be tinged with personal hurt based on neglect.

As president, Carter kept in touch with the man he had unseated, Gerald R. Ford, and ordered regular foreign policy briefings both for Ford and the other living ex-president, Richard M. Nixon. In contrast, Carter received no briefings in the early months after he left the Oval Office. He was upset by this and further irritated by a news item in May, evidently from White House sources, erroneously reporting that Carter was being briefed. It took a threat to "go public" about his isolation, in the wake of the news report, to produce his first briefing, from presidential national security adviser Richard V. Allen late this spring in Plains.

In the months to come, Carter plans to address Reagan's policies in more detail. He will make a foreign-policy speech at a forum already selected, but not announced, toward the end of this year. In 1982 he plans to appear on behalf of congenial Democrats running for Congress.

As for his own future, the first order of business is to complete his presidential memoirs, due at the publisher next spring, and begin work on the Carter presidential library. Another overseas trip, this time to Scandinavia, is likely next year.

After leaving Washington, Carter threw himself into two hobbies which have become important avocations, furniture-making and fly-fishing. Recently he wrote an article about his fishing experiences for a sportsman's magazine.

Beyond this, the former president said he probably will accept an invitation from some university near home to be a visiting professor, and he has "looked into option" of accepting some corporate board positions. The Carter peanut business was sold this year. The farms in Plains remain, under the supervision of son James III (Chip).

"There are a lot of options open to me," said the ex-president. At 56 and in robust health, he can look forward to many active years. He is therefore beginning to turn over in his mind the possibility of still another career after a busy life as naval officer, farmer, state legislator, governor and president.

"I've always had a mission, a driving force in my life, or an ambition, purpose or whatever you call it, since I was a tiny boy. My guess is that one will evolve, but I'm not out searching for some issue."

Carter said he is "competely at ease" about a career decision, believing that he has plenty to occupy his time in the immediate future. Moreover, he has "a fatalistic belief that things will come along." Jimmy Carter is certain there is life after the presidency for him, but he isn't sure just what.