Maxine Kumin, who sings "the music of the middle voice," as one critic said, is the new poetry consultant to the Library of Congress.

She is the 25th to hold the post and only the fifth woman. Winner of the 1973 Pulitzer Prize for her poems of New England, she succeeds William Meredith but will not take over until January. Meredith will serve until then.

Meredith and another consultant, Josephine Jacobsen, were at the luncheon yesterday that introduced Kumin to the Library and to Washington. "A farmer poet," as someone described her, she lives with her husband, Victor, a consulting engineer, on a farm at Warner, N.H.

As a New Englander, she is inevitably compared to Robert Frost, but her nature poems are tougher, perhaps less polite, certainly less conventional. Her images shrink from nothing, even worm-infested manure. One furious poem about cruelty is titled "Heaven as Anus."

There is also quiet humor: the uncle who was "broad as a rowboat"; the lovely picture of her favorite mare lyinging in a summer meadow in the poem "Amanda Dreams She Has Died and Gone to the Elysian Fields."

And loving scrutiny: "At night Amanda's eyes are rage red with toy worlds inside . . ."

And sorrow remembered (from "The Deaths of the Uncles"): but it was Dan Dan Dan the apple of my girlhood with his backyard telescope swallowingthe stars, with the reedy keening of his B-flat licorice stick, Dan who took me teadancing at the Adelphia Club, Dan who took me boating on the Schuylkill scum, Dan who sent the roses, the old singing telegrams and cracked apart at Normandy leaving behind a slow-motion clip of him leading the conga line, His white bucks in the closet and a sweet worm in my heart.

Born in Philadelphia, Kumin took two degrees at Radcliffe and has taught at many universities and colleges from Columbia to Davis and Elkins in West Virginia. She was on the staff at Breadloaf Writers Conference for six years.

Recently she has turned to nonfiction prose, novels and short stories, and she hopes to expand the Library's program of readings to include some short stories.

"I like writing prose," she said. "It keeps you honest: those simple, declarative sentences. It's good discipline after the ellipses of poetry."

She is doing a piece on mules and donkeys and recently published an article about mushrooms.

Many poetry consultants have complained that the Washington pace and the fragmented schedule of lecturing, arranging readings, meeting young poets and being feted left little time for serious work. One insisted he was unable to write a line in his year here.

But Kumin isn't fazed in the least.

"This is going to be a luxury," she said. "I have to write on the plane most of the time. I bet half my poems were written on the backs of boarding passes."

She and her husband will divide their time between farm and city. There are three grown children, gone away. Caretakers will keep an eye on the five horses, the sow, the vegetable garden and cattle, not to mention the two Dalmation puppies that made the news recently when they were dog-napped and recovered.

At 54, Kumin is leaving poetry for the moment (her last volume, "The Retrieval System," came out in 1978) in favor of a collection of short stories. Acclaim is growing. She has been mentioned in the same breath as Rilke. It was Rilke, in fact, whose lines provided the title of her book "House, Bridge, Fountain, Gate," a nice evocation of her directness, her reliance on the power of the simple word itself to transmit emotion, like a secret code.