Edmund S. Muskie of Maine is a man of large intellect and short temper, a senator who enjoys great prestige among his colleagues and a political figure of national stature who has never held an appointed job.

That last fact, friends and colleagues said yesterday, is likely to set him apart not only from Cyrus R. Vance, the man he will succeed as secretary of state, but also from most of the secretaries in modern times. "He's a very different cat than Cy Vance," said one Muskie friend who knows both men. "He is a politician, and he never served on the board of directors of the Council on Foreign Relations." n

This friend and many others agreed yesterday that Muskie will be inclined as secretary to be a stronger presence and a more forceful spokesman than Vance had been.

Muskie was a relatively obscure but popular senator in 1968, when Hubert H. Humphrey chose him as his running mate. Muskie's success as a campaigner made him a national figure, and in the early 1970s he played the role of president-in-waiting to a faltering Richard M. Nixon. But Muskie himself faltered in the 1972 presidential campaign, a failure that sent him back to the Senate, but did not deprive him of his high standing in the Democratic Party.

Though the fact is little known or discussed in Washington, Muskie has a personal relationship with Jimmy Carter that has grown increasingly close in recent time. He and his wife Jane dined alone at the White House with the president and Mrs. Carter on a number of occasions, friends revealed yesterday, and Muskie's appreciation for Carter has grown markedly since a rocky beginning in their relations in 1976 and 1977.

This relationship is also likely to give Muskie greater stature as secretary of state. Muskie has already exploited it, associates indicated yesterday, in frank discussions with the president about his new job, discussions that led to strong, explicit assurances from Carter that Muskie will be his principal associate on foreign policy.

"Brzezinski has got a formidable, formidable opponent," one senior official said yesterday, referring to the president's national security adviser, who often disputed Vance over policy and tactics. "I can tell you which end of the see-saw will be known," this official -- who knows both men well -- observed. He meant that it would be Brzezinski's end.

Muskie is a large figure on the Washington stage, and not just because he is 6 foot 4. His departure from the Senate will considerably weaken the Budget Committee that he has run and brought into prominence since 1974, and it will leave environmentalists without their most effective leader.

Foreign policy has long been a Muskie interest, but never one of his preoccupations. He served on the Foreign Relations Committee from 1971 to 1974, then rejoined the committee last year to help promote the SALT II treaty.

Arms control is one issue on which Muskie has long held strong views. He was an advocate for SALT even before the formal arms control negotiations began, and over the years he has proposed a series of unilateral American moves that he said might slow down the arms race.

But by this year he had become deeply discouraged over the failure of the SALT process to put meaningful controls on the strategic weapons of the two superpowers. In Senate hearings on the SALT II treaty, Muskie elicited an admission from Vance that at the time the treaty would expire in 1985, both countries would have vastly increased their already enormous nuclear firepower.

"I find that a source of great disappointment," Muskie said them. He was gently but firmly critical of Vance for negotiating a treaty that offered no guarantee of ever producing genuine arms reductions. "It seems to me we have to ask ourselves whether or not the urgency of getting an agreement on our side was so great that we yielded to them [the Soviets] on points" that allowed for a continuing buildup, rather than reductions, of strategic forces.

But this year Muskie was one of the liberal Democrats in the Senate who seemed most disillusioned by the Soviet invasion of Afghanistan. Speaking in January of himself and others who supported accommodation with the Soviets over the years, Muskie told Helen Dewar of The Washington Post:

"Betrayed, yes, that's the word. We feel betrayed, used . . . We thought it was in this country's best interests. She [the Soviet Union] just ran out on use."

Muskie said then that the process of getting back on the road to Soviet-American accommodiation would have to start all over again, and that in the future America would have to be more skeptical of Soviet assurances.

When he ran for president in 1972, Muskie's foreign policy positions were classically liberal and dovish. He said the United States should set a deadline for withdrawal from Vietnam; he favored a boycott of South Africa; he opposed troop withdrawals from Europe but favored stricter controls on defense spending.

Vietnam had been a particularly painful issue for Muskie. He was an early supporter of the war, but revealed during the 1968 campaign that he had privately urged President Johnson to stop the bombing of North Vietnam early that year. In 1972 he said he regretted not having opposed the war six earliier. In 1973 he was the floor manager in the Senate of the War Powers Resolution, an attempt to limit presidential power to wage war without congressional approval. a

It is a point of frustration and sensitivity with Muskie that he is perhaps best remembered in public annals for a dramatic outburst of tears during the 1972 New Hamsphire primary, an incident that marked the symbolic collapse of his high hopes in that year's presidential campaign.

Muskie is famous for his temper, but also for his self-discipline, so it was a surprising moment. It happened in a snowstorm on the back of a flatbed truck in front of the Manchester Union-Leader, the rabidly conservative paper that called him "Moscow Muskie" that year and published material critical of his wife. Muskie broke down three times while trying to defend his wife. Television pictures of him wiping away tears became a damning sign of weakness that came to symbolize a presidential campaign that never met the country's expectations.

A senior staff official in the Senate suggested yesterday that one reason Muskie decided to accept this new job may have been his desire to wipe that incident from the national memory. "I don't think he wants to live with the image of the man who cried, because that's not Ed Muskie," this man said.

What Ed Muskie really is depends on who is talking. Some who know him well consider him an awesome figure, but also an agonizing one, a man who has trouble making and sticking to decisions.

It was characteristic that Muskie expressed doubt yesterday that he was the right man to try to restore some order and prestige to American diplomacy. tHe did precisely the same thing in 1972, startling the press and even his own aides on the very first day of his formal campaign by wondering aloud if he was the right man to lead the nation. All during that campaign year Muskie seemed to lack the combination of total self-assurance and conviction that seems to provide the energy for successful presidential candidates.

But friends who have worked with him say Muskie's agonizing is really just an intense desire to get it right, to make sure all facts have been considered before a decision is made. They praise him for boundless energy, great skill as a reader of briefing papers, and a sharp, inquisitive mind.

Diplomacy, friends agree, will be a new experiment for Muskie, by instinct a blunt fellow. He proved that point yesterday at the White House in a private meeting involving President Carter, Zbigniew Brzezinski, and members of the Senate Foreign Relations Committee.

According to one man who was in the room, Muskie was explaining to the senators that he knew he was taking on a tough job, one he didn't expect to hold very long. "It will probably be an eight-month job," Muskie told the senators, apparently a prediction about Carter's reelection prospects.

"When he said that, Carter blanched," this source reported.