WEST PALM BEACH, Florida (Reuters) - Democratic presidential candidate Barack Obama will leave the campaign trail to go to Hawaii this week to visit the ailing grandmother who helped raise him, an aide said on Monday.

"Recently his grandmother has become ill and in the last few weeks her health has deteriorated to the point where her situation is very serious," said Obama aide Robert Gibbs.

Obama's grandmother, Madelyn Dunham, who will be 86 on Sunday, helped raise him along with his mother, Ann Dunham, and his grandfather, Stanley Dunham.

Gibbs would not discuss the nature of her illness.

The candidate is canceling events in Madison, Wisconsin, and Des Moines, Iowa, that had been scheduled for Thursday. He instead will go to an event in Indianapolis, Indiana, on Thursday, then fly to Hawaii to see his grandmother.

He will return to the campaign trail on Saturday, Gibbs said.

Obama often makes references to his grandmother on the campaign trail, mentioning that she worked on a B-1 bomber assembly plant during World War Two. Later, she worked as a secretary in a bank and was eventually promoted to vice president. She helped put Obama through private school in Hawaii.

"Senator Obama's grandmother, Madelyn Dunham, has always been one of the most important people in his life," Gibbs said in a statement. "Along with his mother and his grandfather, she raised him in Hawaii from the time he was born until the moment he left for college. As he said at the Democratic Convention, she poured everything she had into him.

Obama's mother died of ovarian cancer when she was 52.

(Reporting by Caren Bohan; Editing by Doina Chiacu and Philip Barbara)