Barack Obama told Americans their "dreams can be one" if they unite in a stirring new crusade for change, in a riotous finale to the historic Democratic National Convention.

Obama accepted the party's presidential nomination before 75,000 delirious supporters, becoming the first-ever black major-party White House pick, exactly 45 years after Martin Luther King's "I have a dream" anthem to racial harmony.

Evoking King's 1963 march on Washington, Obama said what "people of every creed and color, from every walk of life" heard "is that in America, our destiny is inextricably linked, that together our dreams can be one."

"'We cannot walk alone,'" the preacher cried. "'And as we walk we must make the pledge that we shall always march ahead. We cannot turn back.'

"America, we cannot turn back," the 47-year-old Illinois senator said as he vowed to turn back the turmoil of the past eight years and the "failed presidency of George W Bush."

Attacking McCain

And he tore into his Republican rival John McCain, saying he did not understand the struggles of normal Americans and should stop questioning his patriotism.

"It's not because John McCain doesn't care. It's because John McCain doesn't get it," Obama said.

As thousands of supporters waved tiny American flags after being whipped up into a patriotic frenzy by a pageant of patriotic songs, Obama trumpeted "I've got news for you, John McCain, we all put our country first."

Obama also reflected on the unlikely aspects of his historic White House quest. "I get it, I realize that I am not the likeliest candidate for this office," he said.

"I don't fit the typical pedigree, and I haven't spent my career in the halls of Washington. But I stand before you tonight because all across America, something is stirring. What the nay sayers don't understand is that this election has never been about me, it’s been about you," he added.

"America, we are better than these eight years. We are a better country than this. 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," Obama said.

"We are here because we love this country too much to let the next four years look just like the last eight. On November 4, we must stand up and say 'Eight is Enough,'" he added.

The huge crowd earlier swayed to the soul music of Stevie Wonder and cheered as Democratic heavyweights like Al Gore castigated Bush, pumping up the heat before Obama's appearance.

The Illinois senator, who just four years ago electrified the convention as a mere state lawmaker, also savaged Republican claims that he is not ready to be US commander-in-chief.

"Don't tell me that Democrats won't keep us safe. 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," Obama said.

"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," he vowed.

Nobel laureate Gore directly likened Obama to Abraham Lincoln, one of the greatest US presidents, and aimed several stinging blows at his own old adversary, Bush.

He noted that Lincoln, like Obama was once a state lawmaker from Illinois who was accused by rivals of having too little experience.

"The experience Lincoln's supporters valued most ... was his powerful ability to inspire hope in the future at a time of impasse. 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," said the former US vice president.

Obama also tried to make a connection with working Americans who are having a tough time making ends meet as the economy slumps.

McCain’s VP pick

McCain, meanwhile, had chosen his running mate, a campaign aide said, and rumors were swirling the news would leak out imminently in a bid to sap news coverage from the Democrats.

Minnesota Governor Tim Pawlenty, former Republican presidential hopeful Mitt Romney and independent senator Joseph Lieberman were thought to be high on the Arizona's senator's short-list.

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 jamboree began, Gallup had the race locked in a tie, with both candidates on 45 percent.