Barack Obama will summon America to join his crusade for change, as the Democratic convention climaxes in a historic echo of Martin Luther King's "I have a dream" speech.

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Forty-five years to the day after the civil rights icon spelled out an electrifying vision of racial equality, Obama will claim his prize as the first-ever black presidential nominee of a major US political party.

His Republican foe John McCain meanwhile was finalising his choice of vice presidential nominee, prior to an expected roll out on Thursday, as a new poll showed the first sign of an Obama convention "bounce."

A speech to go down in history

The Illinois senator, after a stunningly swift rise to the pinnacle of US politics, is under pressure to deliver a speech that will go down in history, but must also try to forge bonds with economically bereft heartland voters.

Obama aides have been downplaying expectations: the speech will be a "nuts and bolts" address, said spokesman Josh Earnest.

The 75,000 people who will cram into a football stadium to hear it will reflect the grassroots insurgency the campaign represents, he said.

"The large crowd that we expect is indicative of the campaign that Senator Obama has run from the beginning ... change happens from the bottom up."

Obama's celebrity fans

The 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, will sing 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, will also rock the crowd, as will pop singer Sheryl Crow.

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 Invesco Field football stadium.

Minnesota Governor Tim Pawlenty, one of the last names on McCain's running mate shortlist, said his party was expecting 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.

"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?"

Vice President to McCain

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

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.

Republicans will grab back the spotlight from the Democrats on Friday, ahead of their own convention in Minnesota next week.

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 per cent among registered voters. Before the Democratic jamboree began, Gallup had the race locked in a tie, with both candidates on 45 per cent.

A welcome gate crasher

Obama on Wednesday 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.

He 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.

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."

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

Obama repaid the compliment as he appeared on stage with Biden in scenes that will delight Democratic image makers who have worked to secure an image of unity at the convention.