He has yet to offer a comprehensive endorsement of the new party's presidential candidate.

Al Jazeera's Rosiland Jordan in Denver said tensions remained between Bill Clinton and Obama, with Clinton unhappy at Obama's perceived lack of respect for his record as president.

Biden is expected to  take the lead in attacking John McCain, the Republican candidate, who has moved slightly ahead in the polls following weeks of Republican pounding of Obama.

On Wednesday McCain released a new advertisement attacking Obama's foreign policies.

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In-depth coverage of the US election

In a symbolic gesture, Hillary Clinton will be formally nominated for the candidacy on Wednesday as a way of honouring her and placating some of her supporters who remain angry following a bitter primary battle

"Whether you voted for me, or voted for Barack, the time is now to unite as a single party with a single purpose," the former first lady told a rally of Democrats on Tuesday night.

"Barack Obama is my candidate, and he must be our president," she said.

 

With the US presidential race set to ramp up after the Democratic and Republican conventions, Clinton said the campaign was "a fight for the future. And it is a fight we must win together."