Denver, Barack Obama reached another historic milestone as he took over as the new flag bearer of the Democratic Party with a call to repair "broken politics in Washington" with a sweeping change at a "defining" moment of US history.

"America, we are better than these eight years," said Obama as he formally accepted Democratic presidential nomination to be the first black American in history to lead a major US party, in Denver's open-air football stadium before 85,000 supporters Thursday.

"We are a better country than this," he said, launching an attack on "the failed presidency of George W. Bush" and John McCain, his Republican rival in the White House race on the 45th anniversary of another historic occasion - Martin Luther King's "I Have a Dream" speech.

It was on this day 45 years ago - Aug 28,1963- that standing on the steps of the Lincoln Memorial in Washington, King delivered what is recalled as a landmark in the US civil rights movement: "I have a dream that one day, this nation will rise up and live out the true meaning of its creed: 'We hold these truths to be self-evident: that all men are created equal,' "

In a nationally televised address from an elaborate stage backed by Greek columns evoking memories of that setting, Obama said: "We meet at one of those defining moments-a moment when our nation is at war, our economy is in turmoil, and the American promise has been threatened once more."

"We are here because we love this country too much to let the next four years look just like the last eight," said the son of Kenyan father and a white woman from Kansas whose candidature is seen by some as the partial fulfilment of King's vision.

"On November 4, we must stand up and say "Eight is enough.'" he said suggesting that McCain would continue Bush's policies at home and abroad.

Rubbishing Republican claims that he is not ready to be US commander-in-chief, Obama said: "Don't tell me that Democrats won't keep us safe," Obama said.

"The Bush-McCain foreign policy has squandered the legacy that generations of Americans-Democrats and Republicans, have built, and we are to restore that legacy.

"I will only send our troops into harm's way with a clear mission and a sacred commitment to give them the equipment they need in battle and the care and benefits they deserve when they come home," Obama vowed.

"Tonight, more Americans are out of work and more are working harder forless," Obama said seeking to forge a bond with working class Americans, hammered by foreclosures, high gasoline prices, and soaring food costs.

"More of you have lost your homes and more are watching your home values plummet. More of you have cars you can't afford to drive, credit card bills you can't afford to pay and tuition that is beyond your reach.

"The failure to respond is a direct result of a broken politics in Washington and the failed presidency of George W. Bush," said Obama.

The last presidential candidate to accept the nomination in an open-air football stadium was John Kennedy, who spoke to the Democratic convention at the Los Angeles Coliseum before 80,000 supporters in 1960.

Earlier in the evening, other Democrats attacked McCain's policies with former Vice President Al Gore urging Americans to "seize the opportunity" to change course by voting for Obama as president.

He reminded the audience at the Democratic National Convention of his failed bid to become president in 2000 when, he said, some believed there was so little difference between Republicans and Democrats that it did not matter who won the White House.

"Today, we face essentially the same choice we faced in 2000, though it may be even more obvious now because John McCain, a man who has earned our respect on many levels, is now openly endorsing the policies of the Bush-Cheney White House and promising to actually continue them," Gore said.

"Hey, I believe in recycling, but that's ridiculous," he joked.

Gore suggested the election was close because people feared the change the Obama represents, and compared him and his promise to fellow Illinoisan Abraham Lincoln.

"In 2008, once again, we find ourselves at the end of an era with a mandate from history to launch another new beginning. And once again, we have a candidate whose experience perfectly matches an extraordinary moment of transition," Gore said.

Republicans mocked Obama's lofty oratory and the classically themed backdrop even as McCain took the unusual step of releasing a video, congratulating his rival on his nomination.

"Too often the achievements of our opponents go unnoticed. So I wanted to stop and say, congratulations," McCain said, speaking directly to the camera. "How perfect that your nomination would come on this historic day. Tomorrow, we'll be back at it. But tonight senator, job well done."

A new Gallup daily tracking poll meanwhile showed that Obama was getting the first signs of a lift from the convention, after the race narrowed to a tense dead heat during August.

He led McCain 48 to 42 percent among registered voters. Before the Democratic convention began, Gallup had the race locked in a tie, with both candidates on 45 percent.

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