HONOLULU -- Sen. Barack Obama arrives this evening to spend time with his gravely ill grandmother, Honolulu resident Madelyn Dunham.

Political historians said it is the first time ever a presidential candidate has left the campaign trail so close to Election Day.

Obama's 85-year-old grandmother is suffering from cancer and recovering from a broken hip. She celebrates her 86’s birthday Sunday.

For Obama to leave a presidential campaign just two weeks before an election is unheard of.

"Unprecedented. It's unprecedented. It is something he just has to do," University of Hawaii political science professor Neal Milner said.

Friends said Obama's urgency to get here could be fueled by one of his greatest regrets. That is not being in Hawaii for the death of his mother, Ann Dunham, who died at age 52 of ovarian cancer.

Obama's senior advisor Robert Gibbs said Obama's grandmother is tremendously important to him. Dunham and her husband, Stanley, raised Obama in a Beretania Street apartment.

"She is very sick. We know it comes at a cost on the campaign trail, but Sen. Obama believes his family comes first," Gibbs said.

Analyst Milner said he believes Obama's risk of losing momentum is minimal.

"Here is one reason McCain is not running a very effective campaign now. That is, if McCain was being successful over the last couple of weeks and was continuing it may make a difference," Milner said.

Obama is scheduled to leave Friday night. He will head to Nevada for his next campaign stop on Saturday.