Walter F. Mondale today bid a graceful goodbye to the political world that consumed two-thirds of his adult life, confident that neither he nor his vision of the nation has been humiliated, but worried that "American politics is losing its substance."

"I have no plans for seeking the presidency or any other elective office again," a weary Mondale told reporters as his family and an enthusiastic cluster of aides looked on.

"I've been around, and I think just as you have to know when to get into politics . . . it takes strength to know when it's time to do something else. And I believe, in my life, that time has come. And I face that point with joy and love for Minnesota," said Mondale, disappointed but not humiliated in a landslide defeat.

On the day after that devastating defeat for himself and his party, the Democratic presidential nominee was his own second-guesser.

He acknowledged during a lengthy news conference that "my chances of winning probably disappeared" after his second debate with President Reagan.

Mondale said he was unable to make his case against Reagan, whom he termed "a very popular incumbent," and he conceded that he was hurt by the perception of a good domestic economy, by peace abroad and perhaps by a yearning for "continuity."

Mondale rejected the idea that Reagan's "massive margin" signaled a major realignnment in American politics. He predicted a "strong revival" of the Democratic Party within the next four years -- fueled, he said, by Reagan campaign promises that almost certainly would be broken.

"I believe that one of the biggest meals of crow this administration will eat -- and there will be several -- will be this one concerning revenues," he said of Reagan's pledge to lower the federal budget deficit without raising taxes. "That problem is the equivalent of the domestic hydrogen bomb."

"This is probably a personal judgment . . . and not a judgment on the issues," Mondale said. "I do not think that the American instinct for fairness and these other things has disappeared at all."

For the second day in a row, Mondale carried himself in a style that seemed oblivious to the size of his loss.

After his news conference at the Radisson St. Paul Hotel, he flew to Washington, spending most of the flight moving down the aisle of the chartered aircraft saying farewells to the campaign press corps, many members of which had covered his effort for nearly two years.

The last flight of the "Louisville Slugger," so named after Mondale's performance in his Oct. 7 debate with Reagan in Louisville, ended in style.

With special permission from airport authorities, the plane made a low, slow pass down the Potomac River, over National Airport runway and on up the river, offering Mondale a panoramic view of many of the city's monuments and landmarks, including the White House that he failed to gain.

The plane circled the area and landed, rolling to a stop outside Butler Aviation Terminal. Vice-presidential nominee Geraldine A. Ferraro and her family met the Mondale family outside the plane. The two halves of the Democratic ticket later went their separate ways.

Mondale said he plans to return from vacation in about a week, then go hunting in Minnesota. After that, he said, he will return to private law practice, probably at the Washington offices of Winston & Strawn, the Chicago-based law firm that hired him shortly after he left the vice presidency in 1981.

At his news conference here, Mondale said he planned to "pursue some certain economic necessities . . . . My family has given to me all these years, and they're entitled to something, too." He will keep his voting residence in North Oaks, Minn., he said, but live mainly in Washington.

Mondale opened his news conference in a light-hearted fashion. "Based on an incomplete analysis of the returns, I think there's a good chance that I won't win this election, and I thought I should make that announcement at this time," he said.

Mondale cited several factors for the magnitude of his loss.

"From the very beginning of my campaign for the nomination," he said, "I seemed to have trouble convincing the young Americans and others that I had that vision of the future that I believe I have.

"I was unable to appeal to independents . . . and moderates who were necessary for my victory. Even though I thought my message was a moderate, practical, sound and solid one, it didn't take. I was unable to make the case which I thought was a strong one, and which I think from all my experience was absolutely right, that the long-term tough problems of our nation can only be solved by a president who masters the essential details and who is in command . . . .

"I was running against an incumbent president," Mondale said, "who was very popular personally, who was very well-liked, in the midst of what is perceived as good economic times, and with diminished international tensions -- I think of a temporary nature, but perceived as such -- and with an electorate that understandably was anxious for some continuity. We've had this long history of one-term presidents and I believe the American people were strongly inclined to give the benefit of the doubt to the incumbent on close cases."

During the campaign, Mondale frequently said he had never lost an election when his name was at the top of the ticket. Today, he said this loss had taught him some new things about American politics.

"Modern politics requires television," he said. "I think you know I've never really warmed up to television, and in fairness to television, it's never really warmed up to me . . . . I like to look someone in the eye . . . . I don't believe it's possible any more to run for president without the capacity to build confidence and communications every night."

But, Mondale said, "the thing that scares me about that, the thing that has held me back, is that I think, more than we should, American politics is losing its substance. It is losing the debate on merit. It's losing the depth that tough problems require to be discussed, and more and more it is that 20-second snippet -- you know, the angle, the stick, whatever it is.

"I hope we don't lose in America this demand that those of us who want serious office must be serious people of substance and depth and must be prepared not just to handle the 10-second gimmick that deals, say, with little things like war and peace, but what are you going to do in Lebanon, in Central America, in Nicaragua, in China, in Afghanistan, in Poland . . . . We've got to find people who can handle it at both levels."

Mondale told reporters today that he felt his hopes for victory fizzled shortly after his Oct. 21 foreign policy debate with Reagan in Kansas City, his last hope after a seesawing of political momentum that began a few weeks before the Democratic National Convention in San Francisco in mid-July.

"I knew it was tough from the very beginning," he said. "I felt that our convention may have been the most successful in history. For a while I thought that had changed the landscape, but within a few weeks we were back to about that tough level. After the first debate, I thought we had a chance and things moved again, but then it moved right back to that other level.

"I would say that my chances of winning probably disappeared at the end of the second debate . . . . The president did sufficiently well in the second debate that I think it reassured Americans and they did what I think they were planning to do all along."

Mondale cited the results of the congressional elections as proof that Tuesday's election had not sharply altered divisions in American politics.

"We've had a president who's been reelected by a massive margin while the Democrats actually picked up Senate seats and while, I think, by historic standards he should have picked up two or three times as many House seats as he did," Mondale said. "And I think it says that, contrary to realignment, this is probably a personal judgment for some of the reasons we discussed earlier and not a judgment on the issues.

"The polls all indicated during the campaign that on the issues I was closer to the American people than the president, but for . . . these other reasons they went with the president."

Mondale was critical of the long, bitter primary process that nominated him.

"I have no doubt," he said, "that the intensity of that campaign, the debates, the traveling, the almost constant intensive questions about issues all over this country, prepared me to be a better president when I'd be elected . . . . On the other hand, we went through nearly a year of party bloodletting, of daily attacks on one another. I hated the whole process for that reason and I think it left scars that I carried with me through the rest of the campaign."

But when asked if he thought one of his opponents in the primaries might have done better against Reagan, Mondale chuckled softly and then offered an answer that brought lengthy applause from aides and supporters in the hall.

"I did my best and I worked my heart out and I was the nominee of the Democratic Party and I made my case with all the strength I could," he said. "I believe that I was right. I think I made a strong case.

"I think history will deal kindly with this campaign and with the record that it made. And I think the Democrats made that choice with their eyes wide open. I believe they made the right choice, and I don't want to look backwards."