DENVER - US Democrats on Monday launched their national convention which will make history by anointing first-term Senator Barack Obama as America's first black major party White House nominee.

Thousands of Democratic delegates were called to order for a four-day political jamboree designed to frame Mr Obama as the embodiment of the American dream and to ease divisions opened by the party's bitter primary process.

Earlier, Mr Obama's former primary rival Hillary Clinton made a fresh attempt to rally her millions of primary voters behind the Democratic presumptive nominee, following fresh reports of tension between the two camps.

'Let there be no mistake about it, we are united,' Mrs Clinton said, before the convention which will confirm Mr Obama's stunning rise to the pinnacle of US politics after less than four years in the Senate.

Democrats were also abuzz about the possibility of an emotional reappearance by cancer-stricken liberal icon Edward Kennedy, in what would be a powerful passing of the baton of idealism from his dynasty to the new Democratic hero.

'I imagine nothing would keep him away from a Democratic convention,' said the Senator's nephew Robert Kennedy Jr, who said his uncle, the darling of party conventions past, was still sharp and lucid despite his grim diagnosis.

Another highlight of Monday night in the Mile High City was an appearance by Mrs Michelle Obama, in the first step of an attempt to frame Mr Obama as the embodiment of the American dream who shares the values of the heartland.

Obama spokesman Josh Earnest said the convention, already attracting blanket media coverage with the race to November's election pegged in a dead-heat, would be Mr Obama's best chance yet to tell his story.

'Tonight will be an opportunity for the American people to be introduced to Senator Obama once again, to learn about his values from people who know him best,' he said.

'The story of Barack Obama is firmly rooted in the American tradition.' Mrs Clinton's public bid to soothe inflamed passions left by her bitter Democratic primary battle with Mr Obama, came as polls showed that some of her voters had still not reconciled to voting for Mr Obama.

Ms Clinton later moved on to speak to voters in the Democratic party's crucial Hispanic constituency.

'I am asking you - to those of you who supported me I will be forever grateful - to work as hard for Barack Obama as you worked for me during the primary,' Mrs Clinton said, drawing a standing ovation.

Ms Mannie Rodriguez, a Clinton backer from Colorado, said the former first lady's supporters must unite to defeat Mr McCain.

'Hillary wants us to support Obama the same we supported her and that's what we are gonna do.' Earlier however, senior Obama and Clinton aides aggressively tried to squelch new reports of tension between the two political giants.

'Senator and President Clinton fully support the Obama/Biden ticket and look forward to addressing the convention and the nation on the urgency of victory this fall,' Obama's top strategist David Axelrod and senior Clinton aide Maggie Williams said in a joint statement.

'Anyone saying anything else doesn't know what they're talking about. Period.'

The Politico website reported late on Sunday that 'mistrust and resentments' were boiling between the two camps, and said Obama aides had angered Mr Bill Clinton by trying to limit his convention speech to national security issues.

While the McCain camp was picking at internal Democratic wounds, Mr Obama staged his own cross-party raid by convening a town-hall meeting in Iowa with undecided independent voters and Republicans.

'Right now we got an election to win,' the Illinois senator said at the outdoors meeting in Davenport, in the state that catapulted him into contention for the Democratic nomination in its January caucuses.

'I feel pretty good about my chances, especially now that I've got one of the finest statesmen in this country on my ticket, Joe Biden, as vice president.'

Republicans meanwhile made new attempts to spoil the party, with an advertisement featuring Mrs Clinton saying during the primary battle that she and Mr McCain had a lifetime of experience, while Mr Obama had only a speech he gave in 2002.

'Was she right?' the ad asked, portraying Mr Obama as the most inexperienced party nominee of modern times, and attempting to fan anger among Mrs Clinton supporters at Mr Obama's choice of Mr Joseph Biden as his running mate.

A Democratic official said Mrs Clinton was expected to release her delegates, leaving them free to vote for Obama in Wednesday's symbolic roll-call vote, in a gesture of unity. -- AFP