With the voices of his grandchildren calling out, "Goodbye, grandpa," Sen. Hubert H. Humphrey left Minnesota in the company of the man who now holds the prize he sought for so long.

President Carter, returning today to Washington from Los Angeles, stopped here, just long enough to escort Humphrey and his wife, Muriel, aboard the presidential aircraft that returned them to the nation's capital this afternoon.

On an overcast, Breezy, and cool Minnesota day, the two men stood together at the airport as Carter recited the litany of Americans who owe a debt to Humphrey's three decades of public service: "Every elderly person in our nation, every poor person in our nation, every black person in our nation . . ."

Pale and thin, his voice at times wavering, Humphrey grinned broadly and told more than 1,000 people who say him off that he is feeling much better than that "everything, of course, is relative."

The 66-year-old Minnesota Democrat is suffering from inoperable cancer, and Carter's flight to pick him up was a touching gesture of warmth for the Democratic Party's vulnerable "Happy Warrior."

But in another sense, and not just because of the emotion of the moment, the President's stop here was the highlight of a cross-country trip, giving a lift to Carter's weary traveling party, and perhaps the President himself, after three days in which the high points seemed few.

Carter left Washington on Friday, heading for five states he lost last November before reaching the friendly Democratic confines of Minnesota. He embarked on the journey with his standing in the polls slipping and with an increasing number of questions being posed about his leadership.

Whether he accomplished what he wanted to only he knows for sure.

The purpose of the President's jaunt across the country was never entirely clear, even at the beginning. By journey's end, any sense of focus or central purpose seemed lost in the blur of Carter's public appearances, the sheer volume of the issues facing his administration that he sought to address, if only in passing.

It was in part to be a mobile pep rally on energy, an appeal to the American people to help him extract an acceptable energy progrm from Congress. But his attacks on the oil companies were muted, and the statistics he cited about oil imports and the cost of deregulating natural gas seemed hardly calculated to inspire.

It was in part a nostalgic trip back to the long campaign trail that brought him to the White House. The President returned to Iowa, to the state's Jefferson-Jackson Day dinner, where two years ago he won a straw poll among potential Democratic presidential nominees and finally gained the national media attention he craved.

Carter spoke warmly of Iowa and the people there who helped him early. But he spoke little about agriculture or declining farm prices, and most of what he said on those topics was greeted in silence by 3,000 people in the Des Moines Veterans Auditorium.

Finally, Carter's trip came down to politics, and to that necessity of politics, fund-raising. That brought him Saturday night to Los Angeles and the glitter of a $1,000-a-plate Democratic Party dinner.

It was near the end of a typically brutal day of travel for the President, begun before dawn in the Midwest and ending around midnight in California.

It was a politically important appearance for Carter in a state whose governor. Edmund G. (Jerry) Brown Jr., is considered potentially his most dangerous rival within his own party. Outside the Century Plaza Hotel, hundreds of demonstrators milled about in the service of one cause or another. And a mile away, in another hotel, there was a counter-dinner, its purpose to show displeasure at the President's Middle East policy.

Except for an attempt to reassure his own heavily Jewish audience of his support for Israel, Carter took little notice of any of these or other signs of discontent with his administration. In his soft voice and conversational style, he rambled on about what he seeks for the country, about energy and government reorganization and limiting strategic nuclear weapons and stopping nuclear proliferation.

The audience sat and listend, quietly.

There was one moment when they came alive - when the President told the Californians he was getting up early today to fly to Minnesota and pick up "the greatest democrat of all."

He had said much the same thing earlier, in Detroit an Des Moines, and it never failed to move his listeners. He repeated it in Minneapolis today, with Humphrey at his side, calling him "the greatest American that I know, the No. 1 Democrat in our country."

When his turn to speak at the airport came, Humphrey told how he was called Friday night by former President Ford and how, after some hagging over the point spread, they had bet $5 on the outcome of the Minnesota-Michigan football game Saturday. Minnesota won the game, and Humphrey won the bet.

"I hope that is legal. I'm not sure. If it isn't I ask for forgiveness," Humphrey told Carter.

"I will pardon you on that one," the President replied.

Then the two men and Mrs. Humphrey turned and walked slowly up the stairs of Air Force One, pausing to wave to the crowd one last time before disappearing inside.