The presidential vote cast already by Democrat Barack Obama's grandmother Madelyn Dunham will count despite her dying just hours before election day, a campaign spokesperson said on Tuesday.

"Once you cast an absentee ballot, it counts," Linda Douglass said.

Dunham died in her Hawaii home in the early hours of Monday eastern time at the age of 86 after a long battle with cancer, robbing Obama of the last surviving relative who raised him at the very climax of the campaign.

She had already sent her vote by mail and like all absentee ballots, it will be opened by Hawaii electoral authorities when the Pacific state concludes the nationwide election some six hours after the US east coast has finished voting.

With tears running down his cheeks and his voice choked with grief, Obama lauded Dunham on Monday as one of America's "quiet heroes" and vowed to fight for all such ordinary heroes if he beats Republican John McCain in Tuesday's vote.

AFP