US Democratic party delegates have gathered for a third day at their convention to hear speeches from Joe Biden, the vice-presidential nominee and Bill Clinton, the former president.

Delegates at the convention in the state of Colorado are also to vote on Wednesday to formally endorse Barack Obama as their candidate for November's presidential election.

Biden, the chairman of the senate Foreign Relations Committee, is likely to speak about the convention's Wednesday theme of national security.

Bill Clinton, whose wife Hillary gave her backing to Obama in a speech on Tuesday, has been waging an ill-tempered fued with the Obama campaign for months, and has accused the Illinois senator of playing the race card.

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.

Roll call vote

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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."