Barack Obama will on Thursday summon Americans to join his crusade for political change as he claims his place in history in a dazzling finale to the Democratic convention.

The Illinois senator will formally accept his nomination as the first African-American from a major party to run for the White House before more than 70,000 people crammed into an open-air stadium in Colorado.

"We want to open up this convention to make sure that everybody who wants to come can join in the party, and join in the effort to take America back," Obama said on Wednesday.

Historic echoes will be everywhere: the speech will be taking place on the 45th anniversary of the day when civil rights champion Martin Luther King envisaged a future of racial equality with his "I have a Dream" speech.

Against a classically-themed backdrop, and with fireworks primed to go off after his closing line, Obama will set course for a general election showdown with Republican John McCain in November which polls suggest is a dead heat.

While the evening promises to be spectacular, Obama knows he must try to reach into the living rooms of American voters struggling to make ends meet amid an economic crunch, and who have turned against the Iraq war.

"This speech and the election is not about Barack Obama, it's about the American people and the direction we need to go in, to get us out of the ditch we are in," said Obama strategist David Axelrod.

In a deafening moment of history, the Democratic Party on Wednesday anointed Obama as the first black major-party nominee for president, in a cathartic release of unity, hope and tears.

The newly-minted nominee sent the convention into raptures by crashing his own party a day early, after a stem-winding speech from running mate Joseph Biden and a belated but glowing endorsement from ex-president Bill Clinton.

But McCain - who accepts his party's nomination next week and reportedly settled on the name of his vice presidential pick on Wednesday - is plotting to block Obama's historic quest for the White House.

Obama formally got the nomination when former rival Hillary Clinton halted a roll-call vote and called for the 47-year-old son of a Kenyan father and white mother from Kansas to be nominated by acclamation.

In his first public appearance since his historic elevation, Obama said he was proud to have the Biden family along on a "journey to take America back" and praised the Clintons for helping to heal the wounds of a bruising primary.

Biden lauded Obama, after his own son Beau reduced Obama's wife Michelle to tears by praising his dad's courage after a 1972 car wreck which killed the senator's first wife and daughter.

Biden ripped into an attack on McCain, but paid homage to his rival's heroism as a Vietnam war prisoner.

"These times require more than a good soldier, they require a wise leader, a leader who can deliver change, the change everybody knows we need."

Earlier, tears streamed down the faces of never-say-die Clinton supporters while many African-American Obama backers also gave in to their emotions on a day many thought they would never see.

"With eyes firmly fixed on the future, in the spirit of unity, with the goal of victory, with faith in our party and our country, let's declare together in one voice right here, right now, that Barack Obama is our candidate and he will be our president," Clinton said.

The Republicans meanwhile set the stage for their convention in Minnesota next week, with US news media reporting McCain had chosen his running mate, and would inform him or her on Thursday.

Former president Bill Clinton also offered a resounding endorsement, after harboring bitterness at his wife's defeat.

"Barack Obama is ready to lead America and restore American leadership in the world," Clinton said, his ovation from the crowd recreating his 1990s heyday.

"Barack Obama is ready to honor the oath to preserve, protect and defend the constitution. Barack Obama is ready to be president of the United States," he added.

Clinton had on a previous occasion declined to say Obama was ready to lead, fueling a sense of mistrust between the two camps, which some analysts believe might have severely hampered Democratic hopes in November.

Obama repaid the compliment as he appeared on stage with Biden.

"If I'm not mistaken, Hillary Clinton rocked the house last night," he said. "And just in case you were wondering, I think president Bill Clinton reminded us of what it's like when you've got a president who actually puts people first," he said, pointing to the smiling former first couple.