U.S. presidential candidate Senator Barack Obama steps off the airplane in Hawaii to visit his ailing grandmother in Honolulu on Thursday. ((Hugh Gentry/Reuters))

Barack Obama's "gravely ill" grandmother might not live to see whether her grandson is elected to the White House, the U.S. Democratic presidential candidate said Friday as he took a break from the campaign to visit her in Hawaii.

Madelyn Dunham, 85, who helped raise Obama and is described by the Illinois senator as the "rock" of his family, recently broke her hip. She is also reported to have a heart condition, but Obama has refused to disclose more details of her condition.

"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."

Obama said he doesn't want to risk his grandmother dying before he has a chance to say goodbye, something that happened when his mother died of ovarian cancer in 1995.

"So I want to make sure that I don't make the same mistake twice," the presidential hopeful said in an interview on CBS aired on Thursday morning.

On Friday, Obama's wife Michelle will stand in for her husband at rallies in the Ohio cities of Akron and Columbus, campaign spokeswoman Jen Psaki said.

Obama is expected to resume campaigning in Nevada on Saturday. 

In a campaign ad, Obama described his Caucasian grandmother as the daughter of a midwest oil company clerk, who "taught me values straight from the Kansas heartland" — things like "accountability and self-reliance. Love of country. Working hard without making excuses. Treating your neighbour as you'd like to be treated."

Obama last visited Hawaii in August, when he spent a week on vacation after he clinched the Democratic presidential nomination.

With files from the Associated Press