The Democratic presidential candidate spent about an hour with 85-year-old Madelyn Dunham on Thursday night and then visited again Friday. He was joined by his sister, Maya Soetoro-Ng.
His decision to leave the campaign trail for a day and a half so close to the election reflects the depth of his relationship with Dunham and the severity of her condition.
"Without going through the details too much, she's gravely ill. We weren't sure, and I'm still not sure whether she makes it to Election Day," Obama told ABC's "Good Morning America" in an interview broadcast Friday.
"We're all praying and we hope she does, but one of the things I want to make sure of is I had a chance to sit down with her and to talk to her. She's still alert and she's still got all her faculties. And I want to make sure that I don't miss that opportunity," he said.
While Obama focused on family, he wasn't able to leave the campaign behind entirely.
He traveled the streets of Honolulu, where he was born and spent much of his childhood, in a motorcade of police cars and Secret Service vehicles. A pool of reporters tagged along. Supporters waited in hopes of spotting him for a few seconds.
Even an attempt to take a quiet walk through his old neighborhood involved guards and a crowd of reporters and cameras, attracting attention from passers-by. Obama, clad in jeans, a black shirt and sandals, quickly gave up on the idea and returned to his grandmother's apartment building in an SUV.
About 75 people gathered there Thursday night, including Carole Love, a real estate agent who said Obama's grandmother is "very well-known and well-respected" in the real estate community because of her years of work at a local bank.
Love, who now lives in Texas, praised Obama for taking time away from the campaign to visit Dunham.
"It shows what kind of a person he is," she said. "It's absolutely so fantastic that he stopped everything to come see his grandmother."
Obama was born in Hawaii. His Kansas-born mother and Kenyan father met as college students there, but Dunham and her husband, Stanley, raised Obama for extended periods when his mother lived overseas. He spent years living in the two-bedroom apartment where Dunham is trying to recuperate from a broken hip.
In his memoir "Dreams from My Father," Obama described his grandfather as something of a dreamer. It was his grandmother who was practical enough to support the family by working her way up to vice president at a local bank.
He has often mentioned "Toot" his version of the Hawaiian word "tutu," or grandparent as an example of a strong woman succeeding through intelligence and determination. Many of his speeches describe her working on a bomber assembly line during World War II.
David Mendell, author of the biography "Obama: From Promise to Power," interviewed Dunham in 2004 and concluded that she helped ground Obama and give him a sense of pragmatism.
"You can't underestimate, I think, her influence," Mendell said. "It's only second, I think, to his mother's influence in shaping who he is. His grandfather was a little bit disconnected from the family. But she was really the woman who stabilized the household."
Dunham, who turns 86 on Sunday, has not campaigned for Obama but has followed the presidential race closely, even getting a corneal transplant so that she could see the television better. She appeared briefly in an Obama ad, a frail white woman talking about her black grandson's "depth and a broadness of view."
Obama said the decision to go to Hawaii was easy to make, telling CBS that he "got there too late" when his mother died of ovarian cancer in 1995 at age 53.
"My grandmother's the last one left," he said. "She has really been the rock of the family, the foundation of the family."