For 40 years Muriel Humphrey stood in the shadow of her husband, the supportive wife and indefatigable political partner who never demanded a public identity apart from that of being Hubert Humphrey's wife.

Now this woman of warmth and unpretentious charm is to be a senator in her own right, the most recent in a series of widows appointed to fill Senate seats left vacant by their husbands' deaths.

There is, she said yesterday in accepting Minnesota Gov. Rudy Perpich's offer of her husband's Senate seat, "some important legislative business Hubert hoped to finish.

Many of the Humphreys' friends were unsurprised by her decision, nothing both her dedication to Humphrey and his causes and what one close friend called "her increasing self-confidence and sense of independent identity . . . a kind of creeping effect of the women's movement." They note she had talked about running for the House in 1972, although nothing came of it.

With her small-town tastes and motherly instincts, she was a perfect politician's wife of the old school, complementing Humphrey's "Happy Warrior" flamboyance with her homespun dignity, reserve and steadiness.

But she was also one of the first wives of presidential condidates to strike out of her own in campaigning and, according to friends, she was one of Humphrey's most valued political counselors, especially, as one put it, on "matters of the heart."

After the birth of a retarded granddaughter, she became a crusader for the handicapped, serving on President Johnson's Committee on Mental Retardation as well as promoting many less-well publicized mental health projects. "She's also interested in education, especially vocational and technical education, and has always been interested in anything associated with peace," said Geri Joseph, a longtime political and personal friend from Minnesota.

Joseph L. Rauh. a Washington friend, says he's "always had a sneaking suspicion she was against the [Vietnam] war, "but was "such a marvelous person and wife you'd never catch her saying it."

Asked at a press conference yesterday whether she ever disagreed with her husband, she said: "I don't think that we had real down-to-earth differences. We had discussions, on a lot of subjects, but Hubert was always my guide. I always felt that, you know, he had so much more experience and background in fielding all these questions an understanding them that I felt in the long run he was probably right, even though I might have had a different touch or a different sensitivity about a subject."

Born in Huron, S.D., Muriel Fay Buck met Humphrey over a counter at the Huron drugstore where he worked after dropping out of college during the Depression. They were married in 1936 and she dropped out of college, working at odd jobs (to help him complete college and graduate school) including making sandwiches for him to sell to classmates.

She rang doorbells in his first campaign for mayor of Minneapolis and thereafter began campaigning at his side, and often independently, as he moved from mayor from mayor to senator for the presidency - all the while raising their four children and tending to household chores. Now 65, she also does needlepoint, plays the piano and water skis.

It was 42 years ago that Muriel Humphrey got another call to Washington: a letter from her ten-fiance saying he eventually wanted to run for Congress and seeking her guidance."Together," he said, "we can do things. I am sure." Yesterday she said she had not discussed her taking the Senate seat with Humphrey but added: "I hope he is guiding me today."