He said: "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."
The candidate learned of her death Monday morning while he was campaigning in Jacksonville, Fla. He planned to go ahead with campaign appearances.
Late last month, Obama took a break from campaigning and flew to Hawaii to be with the 86-year-old Dunham, who helped raise him.