DENVER - DEMOCRATS formally named Barack Obama their presidential candidate on Wednesday, putting their hopes of ending eight years of Republican control of the White House in the hands of a man who would be the United States' first black president.

Former rival Hillary Rodham Clinton interrupted a state-by-state roll call vote at the Democratic National Convention to ask delegates to make Mr Obama's selection unanimous 'in the spirit of unity, with the goal of victory'. They agreed, with a roar.

Competing chants of 'Obama' and 'Yes we can' floated up from the convention floor as Mr Obama's victory was sealed.

Though the vote at the Democratic National Convention offered no surprises, its historical importance was undeniable. It capped the longest, closest US primary race in memory as Mr Obama, a political newcomer, defeated Mrs Clinton, the former first lady whose victory once seemed all but assured.

It also meant that Mr Obama, the son of a black Kenyan father and a white American mother, is now one victory from becoming president of a nation where, just decades ago, many blacks were denied the vote.

Mr Obama was across town as the delegates he won in months of primaries sealed his victory. Aides left open the possibility that he would briefly visit the convention to thank his supporters, a routine event at recent national conventions.

His formal acceptance speech on Thursday night was expected to draw a crowd of 75,000 at a nearby stadium.

Mr Obama's prospects in the Nov 4 election are uncertain. He is in a tight race with Republican John McCain, a veteran senator and former prisoner of war in Vietnam, who has attacked Mr Obama for his lack of experience.

Mr Obama has had to fend off questions about his patriotism and rumours that he is a Muslim. And no one knows how many Americans simply will not vote for a black candidate.

Mr Obama also needs to unite a party fractured by the long and bitter primary campaign - a process that received a boost on Tuesday with an enthusiastic speech by Mrs Clinton, who said Mr Obama is 'my candidate and he must be our president.'

The first-term Illinois senator was expected to receive a similar support from her husband, Mr Bill Clinton, who speaks to the convention on Wednesday night. The former president has made little secret of his disappointment, or even anger, over his wife's primary defeat, but aides say he was planning a ringing endorsement of Mr Obama as a leader ready to confront any challenge.

The difficulty in uniting the Clinton and Obama camps showed in the careful negotiations for the delegate roll-call that sealed Mr Obama's nomination.

Mrs Clinton, who won nearly 18 million votes and but could not overcome Mr Obama's delegate total, had wanted the pro forma roll call as a cathartic moment for her huge bloc of supporters.

The compromise, that allowed her to be nominated and the votes to be counted through a partial roll call of states, provided a middle ground and a hope for united Democratic front.

Less than an hour before the roll call, Mrs Clinton began an emotional gathering with her delegates by telling them she had released them to vote for Mr Barack Obama. Many in the crowded ballroom yelled back 'No!'

Mrs Clinton told her delegates she's not telling them what to do, adding: 'You've come here from so many different places, having made this journey and feeling in your heart what is right for you to do.'

Mr Obama, 47, has campaigned on a theme of hope and change, tapping into voter dissatisfaction with the old politics of Washington and the unpopular presidency of George W. Bush. He has also pledged to pull US combat forces out of Iraq in 16 months and to make health care available to all Americans.

He has called for bipartisan unity and targeted western and southern states that have been Republican strongholds. But he is vulnerable in northern industrial states - Clinton strongholds that have been crucial to Democratic hopes.

Mr Obama, his wife Michelle and running mate Joe Biden and his wife, Jill, will embark on a bus tour of three of those states: Pennsylvania, Ohio and Michigan.

Republicans hold their convention next week to anoint Mr McCain as their candidate. He has not yet announced a running mate, but was expected to do so soon.

On Wednesday, Mr McCain's campaign released a new TV ad saying that Mr Obama showed he was 'dangerously unprepared' for the White House when he described Iran as a 'tiny' nation that did not pose a serious threat.

'Iran. Radical Islamic government. Known sponsors of terrorism. Developing nuclear capabilities to 'generate power' but threatening to eliminate Israel,' says the ad, which was being run in key states.

'Terrorism, destroying Israel - those aren't 'serious threats?''

Missing from the ad was the context of Mr Obama's remarks last May in which he compared Iran and other US adversaries to the superpower Soviet Union.

'They don't pose a serious threat to us the way the Soviet Union posed a threat to us,' he said in arguing for talks with Iran. 'You know, Iran, they spend one-100th of what we spend on the military. If Iran ever tried to pose a serious threat to us, they wouldn't stand a chance.' -- AP