Speculation has been rife about who will share Mr Obama's platform
US Democratic presidential hopeful Barack Obama is set to reveal his vice-presidential running mate ahead of a rally in Illinois.
Unconfirmed US media reports say Senator Joe Biden, a veteran lawmaker, is Mr Obama's choice, but he has not yet announced this.
The decision is due to be announced by text message and e-mail to anyone who signs up to receive it.
Democrats start their party convention in Denver on Monday.
In an interview with USA Today newspaper, the Illinois senator said he had selected a running mate who was independent and would challenge him in the White House.
Senator Biden has long experience of foreign policy and national security
He added that he had opted for someone who would help him strengthen the economy, and was also ready to act as president.
But Mr Obama gave no clue as to whether he had notified his preferred running mate yet.
The expectation was that during the course of Friday a text message would be received by those who had signed up for it, revealing the name.
However, by the end of the day there had been no announcement.
Early on Saturday, several US news outlets reported that Joe Biden had been chosen.
Mr Biden has three decades of experience in the Senate and chairs the Foreign Relations Committee - something analysts say would balance Mr Obama's lack of foreign policy experience.
Media crews are camped outside his home to watch for any developments, the BBC's Jonathan Beale reports from Washington.
Surprise in store?
The conventional wisdom is that vice-presidential candidates do not swing elections, correspondents say.
But Mr Obama's choice is interesting because it will reveal a little more about the style of the man and how willing he is to be adventurous.
Mr Obama's rival, Republican presidential hopeful John McCain, has reportedly not settled on a running mate.
Minnesota Governor Tim Pawlenty and former Massachusetts governor Mitt Romney are reported to be under serious consideration for the role.