Washington - Barack Obama's grandmother, who helped raise him in Hawaii, passed away after a prolonged battle with cancer, the Democratic presidential candidate revealed Monday.

Madelyn Dunham, 86, had been gravely ill for weeks, prompting Obama to interrupt his election campaign for a few days last month to visit her in Honolulu, Hawaii.

"She was the cornerstone of our family and a woman of extraordinary accomplishment, strength and humility," Obama and his half-sister, Maya Soetoro-Ng, said in a statement. "Our debt to her is beyond measure."

Speaking late Monday at a campaign rally in Charlotte, North Carolina, Obama grew emotional and wiped tears from his cheek while speaking of his grandmother.

"She was one of those quiet heroes that we have all across America who are not famous," he said. "Their names are not in the newspaper, but each and every day they work hard."

The news of Dunham's passing overnight comes just one day before the US presidential election, which has Obama holding a significant lead over Republican rival John McCain.

Obama was not expected to interrupt his last-minute campaigning and was to appear at a North Carolina rally later Monday.

McCain and wife Cindy issued a statement offering their "deepest condolences" to Obama and his family.

"Our thoughts and prayers go out to them as they remember and celebrate the life of someone who had such a profound impact in their lives," McCain said.

Obama has often paid tribute to his maternal grandmother, and he mentioned Dunham's strong influence when he accepted the Democratic nomination in September.

Obama and his sister thanked supporters for "flowers, cards, well- wishes and prayers" since news of the gravity of her illness was revealed less than two weeks ago, and they asked for Americans to donate to cancer charities. Dunham died "peacefully," the statement said.

Obama was born in Hawaii to a Kenyan father and white mother from Kansas, who were both studying in Honolulu at the time. His parents divorced soon after, and Obama moved with his mother to Indonesia with her second husband. She sent him back to the United States at age 10 to live with his grandparents in Hawaii, where he remained through his teenage years.

"My mother was a single mom, so she raised me with the help of my grandparents. And so my grandmother, my grandfather, my mom, they're really the people who took care of me all throughout my childhood," he said during his visit to Hawaii on October 24.