"It don't mean a thing if it ain't got that swing." What Duke Ellington declared in 1932 holds true today, as, in fact, it always has. The rhythmic imperative of song is absolute, no matter the genre or tempo. A plodding version of "Johnny B. Goode" remains as deadly as a leaden "So What" or a sodden "Vissi d'Arte."

Renee Fleming has an intoxicating voice that has brought her to the pinnacle of the contemporary classical-music universe. But on "Haunted Heart," an intimate recital of popular standards, she just doesn't have that swing. The artistic daring it took to attempt this left-field project can't buy enough goodwill to see a listener through its frequent longueurs. What should have been a joyful journey to revisit her roots (Fleming had early experience as a jazz singer) finds her too often bogged down in overly deliberate interpretations. This is one diva, it seems, who just can't lighten up.

Although featuring superb accompaniment by two accomplished and inventive jazz players, pianist Fred Hersch and guitarist Bill Frisell, and touching on songs that are long familiar in the mainstream jazz repertoire, "Haunted Heart" can't be labeled a "jazz" project. To her credit, Fleming for the most part avoids any cringe-inducing jazzy affectations. Singing an octave lower than usual, she approaches venerable fare such as "My One and Only Love," "You've Changed" and "Haunted Heart," as well as pop standards (Lennon and McCartney's "In My Life," Stevie Wonder's "My Cherie Amour" and Jimmy Webb's "The Moon Is a Harsh Mistress") with economical focus. She could have shown off, displaying her prodigious vocal equipment, but she has the good taste to refrain. There is no slumming here: Fleming obviously loves all this material -- her attention remains centered on the songs at hand.

Yet her reverence ultimately strangles the album. Fleming has an uncomfortable habit of bestowing each word with equal weight, thus dragging down the flow of the performance. The momentum, the glide, that's essential for making a popular song come alive is buried under her often static phrasing.

Strangely enough, veering from her mother tongue dispels Fleming's self-consciousness. Three art songs -- Mahler's "Liebst du Um Schoenheit," Villa-Lobos's "Cancao Do Amor" and Paladilhe's "Psyche" -- find Fleming sounding comfortable and authoritative, firmly in her element.

Still, the album's venture into American grass roots, Stephen Foster's "Hard Times Come Again No More" -- beautifully set off by Frisell's unadorned guitar work -- also elicits a luminous, heartfelt interpretation. It's the kind of chill-down-the-spine performance that makes you hope Fleming attempts a project of this sort again. That is, after she gives a serious listen to, say, Marvin Gaye.

On her new album, soprano Renee Fleming lends her operatic voice to popular standards.