Clinton has said that she will throw her support behind Obama when she formally ends her challenge for the nomination at an event in Washington DC on Saturday.

"I will be speaking on Saturday about how together, we can rally the party behind Senator Obama," the former first lady said in a message to supporters.

Running mate

There is speculation Obama might pick Clinton as his running mate for November's presidential election against John McCain, the presumptive republican candidate.

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Obama has said the choosing of a vice-presidential candidate would take time, and on Thursday Clinton sought to distance herself from efforts by supporters to convince Obama to pick her.

"While Senator Clinton has made clear throughout this process that she will do whatever she can to elect a Democrat to the White House, she is not seeking the vice presidency, and no one speaks for her but her," a statement from her campaign said.

 
"The choice here is Senator Obama's and his alone."
 
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Obama told reporters he appreciated the statement from Clinton's aide deferring to him on the running mate choice.

A three-member team, including Caroline Kennedy, the daughter of assassinated president John F Kennedy, has begun vetting vice presidential  contenders on Obama's behalf.

"Senator Clinton would be on anybody's shortlist," Obama said on CNN television on Thursday.
 

At a rally in Virginia, Obama shared the media spotlight with Jim Webb, the state senator.

Emphatic endorsement

Webb, who had remained neutral as Obama and Clinton battled for the nomination, gave the Illinois senator an emphatic endorsement as he introduced him.

"I'm honoured to stand alongside this man, a man of great intellect who over the past 16 months has impressed all of us as he stood up to sometimes withering attacks with measured responses, unshakable composure," Webb said.

The decorated Marine veteran of the Vietnam war said Obama "has given all of us confidence in the steadiness that we want to see in a commander in chief".

Obama paid triburte to Clinton and vowed to unify the party ahead of the presidential election.

"I know we won't be divided because whatever differences between  me and Hillary Clinton, they pale in comparison to the differences we have with the other side," he said.