Cynthia Nixon as cancer patient in 'Wit'

As everyone knows who has marveled at her creamy-looking contradictions since she first stepped onto Broadway at 14, Cynthia Nixon can do just about anything.

As anyone knows who saw "Wit" as a Pulitzer-winning production Off-Broadway in the late '90sor as an Emmy-winning film on HBO in 2001, this drama by Margaret Edson is an emotionally and intellectually harrowing rarity that explores a cancer death with the profundity, humanity and unexpected humor it demands.

The confluence of this extraordinary actress and play is well worth seeing -- a sensitive, moving revival of drama that's finally on Broadway, where it belongs. So it hurts to have to admit that this is not a perfect match, not the hoped-for combustible fusion of flint and grace.

Nixon -- again challenging herself far from those image-defining years of "Sex and the City" -- seems just too soft to inhabit fully the sharp hollows and hard bones of Vivian Bearing, PhD, the aggressively unsentimental, hyper-analytical John Donne scholar faced with grueling experimental treatment for stage IV ovarian cancer.

The actress, last onstage here in her Tony-winning portrayal as the inconsolable mother of a dead child in the 2006 "Rabbit Hole," is virtuosic at looking as fragile as a girl, almost at the same time she withdraws into a distant and forbidding beauty. Here, with her shaved head covered by a red baseball cap, she is still most convincing in the flashbacks to the childhood moment when she learned she loved words and to the character-forming lesson from her tough mentor (played with beautiful restraint by Suzanne Bertish).

But Vivian, despite the humiliating hospital gown and the IV pole, must first be formidable, a scarily self-contained specialist in a poet who, according to her callow young doctor/former student (Greg Keller), makes Shakespeare sound like a "Hallmark Card."

No matter how hard I tried to see Nixon as her own subtly complicated Vivian, it was impossible to forget the clear-eyed toughness of Kathleen Chalfant in the original, or the hotheaded defensive shell on Judith Light, or, despite the eye-makeup for the vomit scenes, the sharp-tongued audacity of Emma Thompson on HBO.

Director Lynne Meadow, founder and leader of the producing Manhattan Theatre Club, has staged the short hospital scenes and deft flashbacks with clarity and too light a touch. The doctors are especially cruel clunks against a Vivian who cannot keep her lamby sweetness from twinkling under her tough skin.

WHAT "Wit"

WHERE Friedman Theatre, 261 W. 47th St., Manhattan

INFO $57-$116; 212-239-6200; witonbroadway.com

BOTTOM LINE Powerful play, Nixon not tough enough