Barack Obama's 86-year-old grandmother died today, the presidential front-runner said.
Mr Obama suspended his campaign for two days last month so he could travel to Hawaii to spend time with Madelyn Payne Dunham, who helped raise him, after her health deteriorated "to the point where her situation is very serious".
She died today, one day before he could become the first black president of the United States.
As he became the first African American presidential nominee of a major US political party, Mr Obama paid tribute to his grandmother.
"When I hear a woman talk about the difficulties of starting her own business, I think about my grandmother, who worked her way up from the secretarial pool to middle-management, despite years of being passed over for promotions because she was a woman," he told his party's national convention in Denver, Colorado, in August.
"She's the one who taught me about hard work. She's the one who put off buying a new car or a new dress for herself so that I could have a better life. She poured everything she had into me.
"And although she can no longer travel, I know that she's watching tonight, and that tonight is her night as well."
In a campaign advert earlier this year, Mr Obama described his grandmother as the daughter of a Midwest oil company clerk who "taught me values straight from the Kansas heartland".
He said this included things like "accountability and self-reliance. Love of country. Working hard without making excuses. Treating your neighbour as you'd like to be treated".
Mrs Dunham was also the "white grandmother" he referred to in a high-profile speech on race as he called for a "more perfect union" in the US.
Mrs Dunham died peacefully after a battle with cancer, Mr Obama said in a joint statement with his sister Maya Soetoro-Ng.
"It is with great sadness that we announce that our grandmother, Madelyn Dunham, has died peacefully after a battle with cancer. She was the cornerstone of our family, and a woman of extraordinary accomplishment, strength and humility.
"She was the person who encouraged and allowed us to take chances. She was proud of her grandchildren and great-grandchildren and left this world with the knowledge that her impact on all of us was meaningful and enduring. Our debt to her is beyond measure.
"Our family wants to thank all of those who sent flowers, cards, well-wishes and prayers during this difficult time. It brought our grandmother and us great comfort. Our grandmother was a private woman, and we will respect her wish for a small private ceremony to be held at a later date. In lieu of flowers, we ask that you make a donation to any worthy organization in search of a cure for cancer."
The 47-year-old Illinois senator learned of her death this morning while he was campaigning in Jacksonville, Florida.
He plans to go ahead with campaign appearances.