A heavy-hearted Barack Obama on Friday said goodbye for perhaps the last time to the ailing grandmother who brought him up, who he said may not live the 10 days until the presidential election.

The Democratic nominee grabbed some precious time with Madelyn Dunham, 85, in her apartment complex in Hawaii, after stepping off the campaign trail for a day and a half and flying halfway across the Pacific to his native state.

He spent a total of seven hours with Dunham, who he affectionately knows as "Toot" after arriving in Hawaii on Thursday, and was due to hit the campaign trail again in Reno, Nevada on Saturday.

At one stage in the morning, a somber looking Obama emerged from the building for a solitary walk around his boyhood haunts, trailed by a lone Secret Service agent, before spying some reporters and heading back in an SUV.

Obama's decision to quit the White House trail so close to the election raised some eyebrows, but his wife Michelle and vice presidential nominee Joseph Biden picked up the pace.

"I'm still not sure whether she makes it to election day," Obama told ABC News of his grandmother, in an interview recorded before the election front-runner left the mainland.

"We're all praying, and we hope she does... 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 right now."

Michelle Obama said in the key swing state of Ohio that Obama told her he got his "'toughness from Toot.'"

"She taught him with her quiet confidence and that love and support that he could do anything," she said.

Dunham who is suffering from a broken hip, osteoporosis and other undisclosed elements is the last elderly relative Obama (47) has left, after his mother died of cancer more than a decade ago.

Back on the campaign trail, Biden ridiculed McCain's claims to be an agent of reform and said the Arizona senator had been a staunch supporter of unpopular President George W. Bush — who cast his advance vote for his fellow Republican on Friday.

"John McCain is now attacking the Bush budget and fiscal policies," Biden said in a campaign swing through West Virginia, a Republican state where polls show Obama is surprisingly competitive.

"Folks, this is as crazy as the Sundance Kid attacking Butch Cassidy — they were in this together."

Obama was due to hit the road again in the western battleground states of Nevada and New Mexico on Saturday, before heading to another crucial state, Colorado, on Sunday.

The Obama campaign meanwhile argued that they had provided the Illinois senator with multiple routes to the 270 electoral votes needed to take the White House, and cast doubt on McCain's hopes in the key state of Pennsylvania.

Campaign manager David Plouffe said the Obama camp would vigorously engage McCain in the state, which given Democratic strengths in key battleground states has emerged as a must-win for the Republican.

"It's a daunting task, when you kind of wipe away the spin here and look at the facts, in terms of how the vote's likely to come down," said Plouffe on a conference call with reporters.

"In order for McCain to win Pennsylvania, he is going to have to win at least 15 percent of the Democratic vote, 95 percent of the Republican vote, and 60 percent of the independent vote," Plouffe said.

Obama's compassionate leave comes with McCain searching for a sudden shift in momentum and new polls showing the Democrat well positioned in the vital battleground states set to decide the November 4 election.

The Democrat left the campaign trail as a new CBS/New York Times survey found Obama leading McCain 52 to 39 percent nationwide, barely changed from a week ago.

A new sheaf of polls in battleground states by Quinnipiac University suggested Obama maintains a strong lead.

Obama led the Republican in Florida by 49 to 44 percent, compared to a 51-43 percent lead in the last survey October 1, and in another key state Pennsylvania by 53-40 percent, compared to 54-39 percent last time.

McCain lost ground in Ohio, often the decisive state in presidential elections, where Obama leads 52-38 percent, expanding his lead of 50-42 percent at the beginning of this month.

No candidate has been elected president since 1960 without taking two of these three states in the US electoral college that ultimately decides who the next president will be.