Rep. Morris K. Udall, torn between illness and political aspiration, got out of bed during a sleepless night in Arizona last weekend and began drafting a statement saying he would not be running for president in 1984, after all.

But the more he wrote, the more he thought about it. And the more he thought about it, the more the old juices began to flow anew. The warm welcome California Democrats gave him in Sacramento . . . the 8 percent he got in a national poll of Democrats on their presidential preference. He left the statement unfinished and went back to bed.

Mo Udall is sifting, once again, through conflicting advice on his presidential plans. Doctors have offered some tempered encouragement.

They told him that his case of Parkinson's disease should not prevent him either from campaigning for the presidency or serving as president. But they emphasize that this is based only on his "present" condition, which is important because Parkinson's is a disease that progressively worsens. It is an incurable but treatable illness that affects the brain cells that control movement and it has slowed him noticeably.

But his family and friends are counseling him against running.

They will support him wholeheartedly if he runs but they are not enthusiastic about it. In several private sessions in Arizona last weekend, friends told him they are concerned about his personal well-being, his financial well-being and about his tarnishing a shining political reputation with a final but futile campaign.

Parkinson's disease: It obviously looms as a major factor in the Arizona Democrat's decision.

"If I run," he said, "the country will be treated to a national seminar on Parkinson's."

He is not so much worried about the impact of the disease on him as he is its impact on the public.

In 1976, the year he ran second to Jimmy Carter in state after state, he began to experience stiffness and slight tremors in his left arm and left leg.

In 1979 he was diagnosed as having Parkinson's. The disease has caused this former professional basketball player to move like a man much older than his 60 years.

Four experts in Parkinson's disease have examined Udall and his medical records and they have signed a general statement attesting to his present condition. It says:

"His symptoms of mild tremor and stiffness of the left limbs dating back to 1976 are compatible with the diagnosis of Parkinsonism. Since 1979 he has been receiving treatment with Sinemet and Artane and these medications effectively control his symptoms.

"Mr. Udall at the present time shows little, if any, impairment of his functional capacity and there are no medical reasons why he cannot pursue a full and vigorous life."

. . . at the present time . . . full and vigorous life . . . .

The statement, which will soon be made public formally by Udall's office, seems to raise more questions than it answers. Among them: Can Udall conduct a vigorous campaign for the presidency and function fully as president?

"As he is today, absolutely," said Dr. Melvin D. Yahr, chairman of the department of neurology at Mt. Sinai Medical Center in New York who was the chief examining expert called in by Udall's physician.

But he added, "The disease is generally considered progressive. Some patients stay functional for an extended period of a normal lifetime . . . . But I can't read Mr. Udall's future. I'm not a fortune teller. I don't have a crystal ball."

The statement, which is to be issued over Yahr's signature, carries a notation that it has been agreed to and also signed by Dr. Abe Lieberman and Dr. Stanley Fahn of New York and by Dr. Thomas N. Chase, a specialist in neurology at the National Institutes of Health in Bethesda, who is Udall's attending physician.

Parkinson's disease, as Yahr explained in an interview, primarily affects "a selective group of nerve cells in a region of the brain that controls motor [movement] activity."

These cells produce a chemical called dopamine that acts as a special nerve impulse transmitter. The disease causes these cells to cease functioning.

But a drug known as Levo-dopa, or L-dopa, stimulates the remaining cells to produce enough of the transmitter chemical to compensate for the nonfunctioning cells. The drug has enabled Parkinson's patients to continue functioning normally for years.

Sinamet, one of the drugs Udall is taking, is a form of L-dopa. Artane, the other, supresses other transmitting chemicals that would otherwise become excessively active. His doctors say Udall has shown no side effects from the medication.

But the disease continues to spread to other cells in this portion of the brain, even as the medication continues to stimulate the body to produce enough of the vital nerve transmitter chemical.

"And at some point," as Udall's attending physician, Chase, said, "Parkinson's patients become less responsive to the medication."

How long does it take?

"The time factor is extremely variable," according to Yahr. Generally, he said, the disease takes hold over five to 10 years "and slowly progresses so that it takes 20 years to have full-blown symptoms."

In some of the most severe cases, he said, it can spread rapidly in just two or three years.

"At its fullest," Yahr said, "Parkinson's causes the patient to have marked tremor of the limbs, including the head, and marked impairment in movement, so the patient becomes totally dependent physically ." This would include, he said, an inability of the patient to write, and to turn pages.

There is a debate in the medical community, which will become a political issue if Udall becomes a candidate, about whether Parkinson's also eventually causes a deterioration in mental or intellectual functioning.

"There are doctors who would argue that the disease impairs the intellect and the ability to think and function intellectually," Yahr said. He emphasized that he does not believe that the evidence shows this to be the case.

"In the primary form of Parkinson's disease," he said, "there is a good body of evidence that the classical Parkinson's patient tends to remain intact intellectually."

He said there have been patients who suffered from Parkinson's disease and other illnesses, some of whom suffered from senility and loss of mental capability.

But he emphasized his view that there is no evidence that these patients would not have experienced the same loss of intellectual ability even if they had not had Parkinson's.

Udall's doctor, Chase, agreed. "Some doctors would say that 15 or 20 or 25 percent of those who have Parkinson's, patients evidence some intellectual deterioration," Chase said. "But the problem is that Parkinson's comes on late in life . . . usually among people in their 50s or 60s or 70s. Many patients have continued to perform at the highest levels of intellectual ability."

Udall's doctors and political colleagues alike certify that there has been no loss in the sharpness of his intellect, wit or debating skills, which have become his trademark in Congress and on the campaign trail.

His physician, Chase, said he has seen "no pattern of changes" in Udall's symptoms since 1979 because the medication has proved effective. He added that there is no way of knowing how rapidly the progressive illness has been spreading in Udall's case.

"But based on the past, his course is a benign one," Chase said. "And one would expect that for the next few years his course will be a benign one."

Udall, meanwhile, has worked energetically to stay his own political course. His medication has checked the tremor symptoms that the disease brings on. He works out in the House gym, and he jogs.

"There is also a stiffness of muscles that Parkinson's produces--a rigidity, and a tendency to slump over, which I fight rigorously with exercise," Udall told a reporter. "It will take me longer than you to get up out of a chair. But once I'm up, I'll walk as fast as you do.

"Getting to be 60 years old, and having an arthritic lower back is a much more inhibiting factor than the Parkinson's disease."

His experience with Parkinson's has not inhibited him from working the political circuit, nor has it inhibited him from telling what had long ago become one of his favorite commentaries on the nature of his profession.

"There is a dread disease called presidentialitis," he likes to say, "and the only known cure for it is embalming fluid."

Udall said he was well on his way toward licking that presidential bug a few weeks ago, when a poll taken for political consultant David Garth primed his pump anew. It showed Udall coming in third as the choice for president among the nation's Democrats. His 8 percent showing was far distant to Walter F. Mondale's 42 percent and John Glenn's 18, but it stunned him nonetheless, considering he had not even begun to run.

"My 8 percent in the Garth poll showed me something," Udall said last week. "And so did the reception in Sacramento [at the state Democratic convention] ."

Udall and his closest advisers conceded that there is no way of measuring how much of that warm applause in California was presidential support and how much was personal sentiment. But Udall added:

"You can't deny there is a warmth there. It's very impressive and flattering to me. These are cynical types who've been around."

The statement from his doctors has given him new political life, he believes. "That report is crucial," he said. "If the doctors' report had been negative, it'd be fold your cards."

This past weekend, he discussed the prospects of a new Udall candidacy with his wife and friends in Arizona.

"My wife's state of mind is that she hopes I won't go, but that she'll go if I go," he said. And in several sessions with friends, he found "much more sentiment agreeing with my family. They were telling me, 'You've had great honors and great support, but you don't need to do it.' "

His friends point out that several other Democratic candidates share Udall's basic liberal beliefs so that there is no philosophic imperative for him to run.

One of Udall's friends, Phoenix attorney John P. Frank, who set up one of the meetings Udall attended, said there was considerable question whether Udall could raise enough funds in Arizona to start a presidential campaign, since that state's Democrats were just about tapped out in costly campaigns last year.

"We decided it was a fair thing to let an exploratory committee tap all possible sources and see what it comes to," he said. "But there was no instant enthusiasm or optimism.

"This fund-raising thing is interrelated with the Parkinson's disease question. People will want to know: Is this the last lance of a wobbling Don Quixote or is it a real run for the roses? There is a concern over whether the contributors will feel they are contributing to a live one or not."

Another of Udall's friends, attorney William P. Mahoney, who served as ambassador to Ghana in the Kennedy years, said that while Udall's friends spoke of reservations about his health and financial prospects, "we assured him that if he makes the run, he has our full-blown, unconditional support. I'd do anything in the world to help Mo. He'd make a superb president. But I'd like to see Mo stay where he is. He's earned his spurs."

Another friend referred a reporter to an interview that Udall gave in 1976 in Evanston, Ill., campaigning on behalf of the Carter-Mondale ticket, having fallen short in his presidential run that year.

Udall spoke of politicians infected by "presidentialitis."

He said then: " . . . I think I can resist . . . . I don't think I'm infected with it." He recalled that he had ended his professional basketball career as a backup forward with the old Denver Nuggets at just the right moment--after a game in which he scored 20 points. "My health was good, I was 24, and I had command of the game.

"But I walked away."