Barack Obama will Thursday warn that eight years of "broken" politics and "failed" Republican leadership is enough, and issue a call for sweeping change at a "defining" moment of US history.

"America, we are better than these eight years," Obama said, in speech excerpts released before he formally accepts the Democratic Party's charge to become the first black major party White House nominee in history.

"We are a better country than this," he said, launching a withering attack on the Bush administration, and Republican White House candidate John McCain.

"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 will tell a 75,000 strong crowd at an open-air football stadium in a dazzling finale to the convention.

"We are here because we love this country too much to let the next four years look just like the last eight," Obama said.

"On November 4, we must stand up and say "Eight is Enough.'"

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," 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.

Speaking in historic echoes on the 45th anniversary of Martin Luther King's "I have a dream speech," Obama also sought to forge a bond with working class Americans, hammered by foreclosures, high gasoline prices, and soaring food costs.

"Tonight, more Americans are out of work and more are working harder for less," Obama said. "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.

McCain meanwhile had chosen his running mate, a campaign aide said, and rumors were swirling that the news would leak out imminently in a bid to sap news coverage from the Democrats at the end of their four-day convention.

McCain also 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."

The ad's tone was a throwback to the kind of "respectful" campaign promised by McCain before the White House battle headed for the gutter. But it also might make Obama look churlish if he criticizes his rival too harshly.

Obama's star-studded pageant of glitz and patriotism will be whipped up by Motown icon Stevie Wonder. Jennifer Hudson, a former "American Idol" contestant who won an Oscar for the musical "Dreamgirls," sang the national anthem.

Singer will.i.am, who wrote a song based on Obama's "Yes we can" speech that became a popular music video, also rocked the crowd.

But Republicans mercilessly mocked Obama's lofty oratory, and the classically-themed backdrop that some observers have compared to a Greek temple set up at the Denver Broncos football stadium.

Minnesota Governor Tim Pawlenty, one of the last names on McCain's running-mate shortlist, said here the Democrats were sure to put on an "eye-popping" spectacle.

"The real point is does this just feed into the growing perception that what we have is one big glitzy production?" he told reporters here.

"But really, after the crowd files out, after the fireworks take place ... the question remains, what is left and is this person ready to be president of the United States?"

McCain was expected to tell his vice presidential nominee of his choice on Thursday, before making a debut campaign swing appearance with the pick on Friday.

As well as 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.