SHE IS AMERICA'S ranking musical comedy star. What makes Carol Channing tick?

She had none of the sprite-like elan of the "20s' Marilyn Miller. She doesn't belt a song like the '30s' Ethel Merman. She has none of the demure sweetness of the '40s' Mary Martin.

Yet, there she is, one of the most instantly recognizable faces, voices and personalities of the nation, a unique image.

I first saw her at New York's Broadhurst Theater in a little revue, "Lend an Ear." She wasn't one of the leads. Yet, it was that tall, goggle-eyed blond that you looked at curiously, persistently. She had that quality which made Ethel Barrymore insist that 20 seconds are all that is needed to tell whether or not a novice can make a career.

That particular charisma led to the part which put Channing on the map, Lorelei Lee, the not-so-dumb blonde of "Gentlemen Prefer Blondes." Anita Loos had described a tiny, seductive girl in whose hand a gun had "just happened to go off and shot Mr. Jenning." Channing was not tiny (actually she's 5-feet-10) but, as she explained, "I thought tiny."

The image captivated the nation, and by the time Marilyn Monroe got the movie part, Channing had toured the land so thoroughly that she could always count on a welcome everywhere she'd been.

This is part of what makes Channing tick. Like the Lunts and Helen Hayes, her good friends, Channing created a national following by the dogged route of getting around. None of her musical comedy peers liked leaving New York to tour the boondocks. To Channing that's downright fun. Not once has she missed a single one of her thousands of scheduled performances.

Another aspect is that Channing performs in all fields - TV, clubs, films, recordings. She's not known strictly as a stage performer, thus her audience is broad and deep. Those who see her in one medium want to watch her in another.

This versatility gave her the key to "Hello, Dolly!" in which she returns this week to the National Theatre, where, 15 years ago, a critical change at the end of Act I turned a fair musical into one of the American musical stage's classics. Others have played Dolly Gallagher Levi, indeed even Dolly Levi Gallagher as a film accommodation to Barbra Streisand, but none have come close to the original.

Some have taken it purely as a vehicle, a range of slightly gone movie stars who've used the waiters' welcome in Honoria Gardens and the title song as a personal tribute. Several stars have roamed about in tacky versions, demeaning Jerry Herman's melodic score and Thornton Wilder's social criticism: "Money it like manure. I like to spread it around and make things grow." Michael Stewart, who adapted Wilder's "The Matchmaker," used the widow's scenes addressed to her dead husband to take the curse off the character as a money-chaser.

From the start, Channing saw as the character's key those moments when, on the runway stepping out into the audience, she talks to Ephraim. Those were the moments, she has said, when Dolly had to be believable, human.

She'd learned by playing the night clubs how to make swift, intimate audience contact. The razzle-dazzle was simple to do but her successors never realized how vital the Ephraim scenes are.

Thus, intelligence is one of the major reasons for Channing's survival in the Miller-Merman-Martin field. The daughter of George Channing, a leading reader for Christian Science - and onetime editor of that church's Monitor - Channing is strong on education. She went to Bennington and a few years ago gave its commencement address. When, probably because of her friendship with the Kennedy clan, she wound up on "the dean's list" of the Nixon administration, she was able to crack: "My son has just made the dean's list at Williams and I'm terribly thrilled to have made the dean's list at the White House."

Within Channing, too, is a slavish devotion to duty. She neither smokes nor drinks and in time has become outspoken about her allergies, which require special organic foods that she carries about in silver thermos bottles. Food is shipped to her weekly on tour, air-mail, at considerable expense. "Aren't I lucky that I can afford it?" says Channing.

Like Carol Burnett, Mary Martin, Mary Tyler Moore and MitziGaynor, Channing benefits by having a savvy husband who also manages her career. Once the producer for Burns and Allen, Charles Lowe serves as Channing's "magnificent no" man, creating schedules, putting variety into the career and smoothing the way for that concentration the superstars must have. Their son, Channing Lowe, is cartoonist and writer for the Daily Oklahoman.

Despite the nomadic life - home is in Sherman Oaks, Calif. - it doesn't take 20 minutes for the Lowes to settle into each city. In their luggage are framed photographs of family favorites, books, a record-player, discs and small mementos which swiftly transform impersonal hotel rooms into a highly personal home. By this time tomorrow the Sheraton-Cariton's presidential suite will look as though the Lowes have lived there for years.

"Tired of playing Dolly? Oh, my no," murmurs Channing. "Every audience is different, and to this generation its' all new. This revival, produced by the Houston Grand Opera, makes far more of jerry Herman's score than the original version, which, I remind you, originally didn't have an overture. The opera people asked Jerry to create one because they see it as one of the greats in the line of American musicals. So this time, with far better voices, even mine in better shape, the score gets more of its due."

Channing's attention to detail is reflected in the final "Dolly" scenes. Originally when she began a song with "I snuggle up to my cash register," she was wearing bridal white. "It made Dolly a gold-digger, which she's not," explains Channing. "So I asked Mr. Mustache (producer David Merrick) if I could wear a more business-like blue.

"That was fine with him, but he said, "Here we've spent a fortune on that white bridal outfit. What about wearing that one for the encores? Think you could make a fast change?' Well, you know I could."