WASHINGTON — A powerful podium like Tony Snow's and wealth like Elizabeth Edwards' mean nothing to cancer cells. A disease that afflicts the comfortable along with everyone else, cancer casts a large shadow in this presidential campaign, and now in the White House.

The back-to-back revelations that President Bush's spokesman and the wife of Democratic presidential candidate John Edwards are battling recurrences of cancer have resonated not just in living rooms but high places, too.

GOP presidential candidates Rudy Giuliani and John McCain have had the disease, in serious form. The past three presidents were touched by cancer, both personally and in their families, and the last chief justice died of it. Members of Congress mourned one of their own last month, killed by it.

Partisan opposites, Edwards and Snow share a struggle against a disease that kills more than 500,000 Americans a year and leaves no one of extraordinary or common means immune.

"Disease in general and cancer in particular is a great equalizer," Dr. Durado Brooks, director of prostate and colon cancer programs for the American Cancer Society, said in an interview. "Any disease, including cancer, will quickly humanize you and equalize you."

It was Snow who offered best wishes from the White House last week when the Edwardses announced a return of her cancer and their decision to press forward in the Democratic presidential race. "Good going; our prayers are with you," he said.

On Tuesday, John and Elizabeth Edwards were quick to offer praise and prayers for him.

Snow, 51, had his colon removed in 2005 and underwent six months of chemotherapy after being diagnosed with colon cancer at an advanced stage. On Monday, doctors removed a malignant growth from his lower right pelvic area and discovered cancer had spread to his liver and more parts of his body.

Elizabeth Edwards probably faces chemotherapy for the rest of her life, now that cancer found in her breast in 2004 has come back, in her rib and possibly elsewhere. In disclosing her incurable condition, she acknowledged she will get a quality of care that many others with breast cancer do not.

"You know, is this a hardship for us?" she asked. "Yes, it's yet another hurdle. But I've seen people who are in real desperate shape who don't, first of all, have the wonderful support that I have, and have no place to turn."

Heart disease — the leading killer — appears more sensitive than cancer to income disparities, with one new Duke University Medical Center study finding that the poorest Americans were more than twice as likely to die from it than patients with more money.

An American Cancer Society study found less dramatic but still marked differences among cancer patients — a five-year survival rate that was 10 percentage points lower for people in the poorest counties studied, compared with those on the next rung up. Such disparities vary widely according to cancer type.