One day last week doctors attending Hubert H. Humphrey got one of their first signals that he was truly sinking. He gave up talking on the telephone.

Until then - in a 10-year battle against bladder cancer - he had, in the almost identical phrase of everyone close to him, "always bounced back."

"I'm Hubert Humphrey," he said, "and all those damn statistics don't mean anything when it comes to me." He knew very well he might not win, but he maintained, "That's the way you've got to feel."

Until last week. Then, last Tuesday or Wednesday, those around him noticed that he was no longer telephoning President Carter, Secretary of State Vance, ex-Secretary Kissinger, "everybody."

The physically diminished Humphrey had finally lost his battle against cancer. But as a human being, he was never diminished, say all those who surrounded him in his last weeks.

"He was an unbelieveable man, in sickness and in health," said Dr. John Najarian, the noted surgeon who cured for him during the final days at his Minnesota lake home, in a room fitted out like a hospital.

"Every day, just until the last days," said Najarian, "be'd struggle to get himself out of bed in the morning. He'd say, 'Today I'm going to walk further than I did yesterday.' He'd walk a little ways, then collapse.'"

"It was shocking to see him up close," said economist Walter Heller, who visited him on Jan. 7. "He was so emaciated. But that wonderful Hubert Humphrey spirit was still there."

He even joked. "You can cut back on the funeral arrangements," he said an aide, "because all the eulogies already have been delivered."

until the advancing cancer finally are into many vital organs, the Minnesota senator and former Vice Prexident read, dictated, conducted some senatorial and personal business, visited his family and staff and friends - and talked.

He talked much to his wife, Muriel, who has been under recent treatment on heart disease and the pain of angina.

And he talked on the phone to Carter, Vance, Kissinger. About the Middle East (urging steady but "not aggressive" U.S. pressure to keep up to momentum toward peace). About blacks and education and jobs (urging the President to back the Rev. Jesse Jackson's PUSH for Rxcellence program among young blacks).

About jobs and the economy. He advised the President not to reappoint Arthur Burns as Federal Reserve Board chairman because - though Humphrey personally liked Burns - he was "always" sniping at Carter and his policies had neither defeated inflation nor generated enough jobs.

He was the practical politican. To Jesse Jackson, who visited on Jan. 7 to voice thnaks for his support, he said, "You've got to get labor people behidn you. And get the religious people to move too. Both labor and religion have plenty of money.

"Mr. Humphrey may be dying," Jackson later commented, "but he knows how to get things done."

An impressed Dr. Najarian told a friend in early January: "What stimulates him most is talking to people. That's where he lives. This man thinks government. This man thinks politics.

"Amazing as it was," said Najarian last week, "no matter how sick he was, he was very seldom preoccupied with himself."

Constantly - as he promised violinist: Isaac Stern, an old friend and political supporter, in mid-November - he vowed, "I'm not going to quit as fast as they think I will."

BUt he was really no Pollyanna. He also told Stern, "I won't be able to beat it altogether."

He candidly told Robert Byrd, after the West Virginian beat hin for the Senate majority leadership one year ago, "I don't know how much time I've got around here."

Over the years he underwent several courses of chemotherapy with powerful anti-cancer drugs that poisons not only the cancer but much of the body. "I feel I've been hit by a truck," he sometimes said, and he sometimes called it "that damned chemotherapy!"

He wrote last summer, "The worst moment in my life was when I discovered I had cancer." And "there are days when I get discouraged."

He wrote in Reader's Digest last August that "one time" - when X-ray treatments caused bladder spasms in 1973 - "I was in such agony that I honestly wanted to give it all up." He actually wished for death then, he told an old friend.

But Najarian said of his recent, far harder struggle, "I've never seen a man who fought harder" until "he finally got so weak he had to give up."

Even then it was more a matter of his body giving up on him.

The body's battle took a decade. It was in June, 1967, that some red blood cells were first found in his urine. The diagnosis was only a mild cystitis but, as his then physician Dr. W. Dabney Jarman said later, that was really "the begining."

A year later an examination showed what was then termed merely "dysplasis," an abnormality of the bladder lining. It was not until the early 1970s that most doctors began calling the same condition "in situ carcincoma," or cancer cells in a limited site, without spread.

There is still medical disagreement about whether to operate without delay. Operating means removing the entire bladder and other structures, causing impotence and requiring an artificial opening for urination. Many surgeons still hesitate to urge so radical a step prematurely.

In 1972, however, Humphrey's doctors began using anti-cancer chemicals. In 1973 they treated a more "potentially cancerous" change with X-rays, after four consultants recommended this course, three recommended no treatment at all and two urged prompt surgery.

If he'd had the surgery," one doctor familiar with the case said last week, "I think he'd still be alive. BUt we can all look back on some cases and say what we should have done."

After the consultants' vote, Humphrey himself made the final decision against surgery. "He always likes to put everything to a vote," his personal doctor explained.

When all these details first became public in early 1976 and the potential presidential candidate was peppered with reporters' questions, he once said with some irritation: I'm a darn sight healthier than some of the reporters who keep asking me about my health."

And, "I think what's most important about a person is what's in his head, not in his bladder."

But in fall 1976 doctors found that the mere "potential" cancer of 1973 had now become frank and unmistakable and invaded his bladder wall.

It had also reached his nearby lymph system, the chain of glands that help fight infection but can also help spread cancer, When Dr. Willard Whitmore of Memorial Sloan-Kettering Cancer Center, New York, removed Humphrey's bladder that October and found that kind of spread, it meant statistically, he had only a one in five chance of surviving five years.

Last August, University of Minnesota surgeons under Najarian found the cancer had spread so massively it could no longer be removed at all.

The Minnesota senator was never fooled. A day after that aborted operation, he asked Najarian what the surgeon calls "the hard questions." He got the hard answers.

In November and December at the National Institutes of Health in Bethesda, he was hospitalized for more chemotherapy. "It was a rare occasion when he was the slightest depressed," said Dr. Vincent DaVita there. "I always came away from hin buoyed up. And I'm supposed to be the one buoying up the patients."