In focus

In-depth coverage of the US presidential election
The Illinois senator said he has decided on a running mate who will be prepared to step in as president, can help him govern and will be independent.

"I want somebody who is going to challenge my thinking and not simply be a yes person when it comes to policy-making," he told CBS' "The Early Show".

Those mooted for the position include Joe Biden, senator for the US state of Delaware and known for his foreign policy expertise, and Bill Richardson, governor for the US state of New Mexico.

Convention speech

NBC News, quoting sources close to the campaign, said on Friday that two other people believed to be prime contenders - Evan Bayh, senator for Illinois, and Tim Kaine, governor for Virginia - had been informed that they would not be picked.

Obama's former rival for the Democratic nomination, Hillary Clinton, Texas congressman Chet Edwards and Kathleen Sebelius, governor of Kansas, have also been mentioned as possibilities.

Democrats are preparing for the national convention in Denver [EPA]
The move comes as the Democratic party is preparing for its annual convention in the city of Denver in Colorado next week, where both Obama and his vice-presidential choice are due to speak.

Meanwhile senior Republican officials say that Obama's Republican rival, John McCain, has not settled on a choice of vice-president, although Mitt Romney, former governor of Massachusetts and Tim Pawlenty, governor of Minnesota, were being considered.

However it is likely McCain will wait to see who Obama selects before picking his running mate.

Obama was happy to let the vice presidential speculation linger as his campaign focused on another issue, McCain's inability to tell a newspaper how many houses he owns. 

The Democrat said that the incident was evidence that the Arizona senator was out of touch with the economic struggles of most Americans.

For his part, McCain continued his attacks portraying Obama as nothing more than a celebrity, with an advert announcing: "Celebrities don't have to worry about family budgets, but we sure do''.