Former president Jimmy Carter gamely acknowledges that his endorsement of Walter F. Mondale could be a burden.

"There is a lot of baggage that goes with me and my administration," he told reporters at a breakfast yesterday.

He had returned to Washington, the town that he conquered and that eventually conquered him, to promote his presidential memoirs, "Keeping Faith."

Mondale has been quietly seeking a kind of "no-fault" divorce from his old boss, something that his fellow Minnesotan and former vice president, Hubert H. Humphrey, never could negotiate from President Lyndon B. Johnson.

For the first time, Carter indicated that he might go along.

Obviously, he has been brought to realize that his attacks on Sen. Edward M. Kennedy (D-Mass.) hurt Mondale, not Kennedy.

Going out among Democrats, as he did during the recent campaign, Mondale found that they look back in anger at the Carter years. Just why this is so is not entirely clear. Carter started no war and presided over no Depression. Camp David was high statesmanship; Democrats are proud of Carter's human rights policy.

But for some reason -- maybe because he started out telling the American people that he loved them and ended up telling them that they had "malaise"--they do not wish to be reminded of him. Pressed for the roots of their wrath, they cite some detail such as his early and expensive concession before polls closed in the West on election night in 1980.

Carter has lately fed their rancor by attacking Kennedy whenever the opportunity arises. They have retaliated by indicating in the polls that Kennedy is their preference for the Democratic nomination. Mondale does not appreciate being used as the blunt instrument for Carter's revenge. To prove the point, he went to Boston to campaign for Kennedy.

The matter of why Carter lost the election is not scientifically established. Some think that 1980 was a referendum for revolution, a primal scream of exasperation against big government spending for the undeserving unfortunate. Others see it as rejection of a self-righteous southerner and his "yes-no" style of leadership.

Carter, however, has no doubts about what happened. He was done in by Kennedy.

Recently, Barbara Walters asked Carter whether he was "mean or mush." He took care of "mush" by declaring that if Ayatollah Ruhollah Khomeini had harmed a hair of the hostages' heads, he would have attacked Iran. But "mean" was not so easy. He repeated his belief that Kennedy is not a man to be trusted.

Carter is in the grip of an obsession about Kennedy. He has been in Plains for 18 months, writing his book on a word processor. He says he has "an interesting life" and grand plans for the future. But Plains has more gnats than people, and some consider it an ideal place to brood over lost glory, perhaps not quite in a class with Belfast but rich in possibilities for vindictiveness and self-pity.

And Carter has concluded that if it had not been for Teddy and the clumsy, but divisive effort to unseat him, he and Rosalynn would still be in the White House, fighting liberals instead of gnats.

Carter looks well; his hair is darker and less fluffy than when he was the leader of the western world. He speaks with his usual command of facts and statistics. He is, even now, infinitely better informed than his successor in the Oval Office.

While reiterating his preference for his former vice president, Carter said yesterday that "others would be well-qualified" and mentioned a fellow southerner, Florida's Reubin Askew, who is as obscure as was Carter when he started his quest for the White House. He also spoke of Sen. John Glenn (D-Ohio) and his "Eisenhower demeanor."

"Obviously," he said with quite uncharacteristic dryness, "Fritz, in his political wisdom, wants to separate himself from me."

Carter had strong language for Mondale's recent suggestions of protectionism in trade with Japan. He called them "vituperative attacks." Mondale must be grateful. Anything that divides is welcome.

Carter might be better off with Democrats if he took aim at Reagan instead of Kennedy. Reagan invites criticism that Carter is especially qualified to make. Reagan has trashed Carter's human rights policy, most notably in Central America where Carter introduced the subject. This week, Mrs. Reagan entertained the wife of Chile's infamous president, Gen. Augusto Pinochet, at tea. And the president muzzled our ambassador to El Salvador, Deane R. Hinton, a career diplomat who finally exploded at the thugs running the government we support.

But Carter never mentioned human rights. He never did know his strongest suit. If he's bitter, he's entitled. Any ex-president finds it hard to concede that his help for a favorite candidate could be "a handicap."