DENVER -- The Democratic Party on Thursday aims to mark a new era, as Barack Obama accepts the presidential nomination before a crowd of more than 70,000.

His address comes after Joe Biden stepped in to his role as running mate Wednesday night with a scathing attack on John McCain and the Bush administration.

The apparatus of the Democratic National Convention will move up the street to Invesco Field at Mile High for the final day of festivities, operating under the campaign's theme of "Change You Can Believe In." Obama, known for his soaring rhetoric, is expected to use his speech to convey a simple message about how he would help his country as president.

"Senator Obama has a job to do out there tonight," spokesman Bill Burton told FOX News, "which is to go out there and let the American people know where he comes from, where he wants to take this country and lend some specificity to what exactly his plans are when it comes to tax cuts and health care and responsibly ending the war in Iraq. He's going to talk about all of those things in pretty clear terms."

The pageantry of the event -- with an antiquities-inspired colonade on the field and several musical acts, including Oscar-winning American Idol alumna Jennifer Hudson singing the National Anthem-- has already earned the nominee Republican criticism of Obama as a celebrity without substance.

"It looks like they are getting ready for the emperor to arrive don't you think?" asked Minnesota Gov. Tim Pawlenty, one of few on McCain's vice presidential short list. "And it feeds into the notion of it's a big production, a lot of entertainment, a lot of hoopla and I think with the Roman columns and the facade it is perfect because the facade is for production purposes but there is not much behind itand that's a perfect metaphor for Barack Obama's readiness to be president of the United States."

The Illinois senator enters the momentous day as the first black nominee of a major party, and after receiving two full-throated endorsements from Hillary and Bill Clinton. John Kerry, the 2004 presidential nominee for the Democrats, also renewed his support for Obama on Wednesday night.

With Obama taking the helm of the party, his newly minted vice presidential candidate Biden, a Delaware senator, slammed the Bush administration and linked John McCain to an entrenched Iraq war policy.

Biden was joined at the end of his speech by Obama -- but not before Biden did Obama's bidding, faulting the Bush administration for losing the nation's prominence in the world and saying a McCain presidency would bring four more years of the same.

"Our country is less secure and more isolated than at any time in recent history," Biden said. "The Bush-McCain foreign policy has dug us into a very deep hole, with very few friends to help us climb out."

Biden, who has a penchant for verbal gaffes, made a slip of his own, but one that earned wild laughter as Biden mistook President Bush for McCain.

"You know, folks, that's the America that George Bush has left us. And that's the America we'll continue to get if George - excuse me, if John McCain is elected president of the United States of America. Freudian slip. Freudian slip," he said.

Biden's speech followed a dramatic day in which Obama's vanquished rival, Hillary Clinton, took to the floor of the convention center to put an end to the roll call vote and move for the Illinois senator to be nominated by acclamation. That was followed up by a rousing speech delivered by former President Clinton, who forcefully offered his and his wife's support for the Democratic team.

At the end of the Biden acceptance speech, Obama walked out on stage.

"I want everybody to now understand why I am so proud to have Joe Biden and Jill Biden and Beau Biden and Mama Biden and the whole Biden family with me on this journey to take America back," Obama said.

Biden drew sharp contrasts between his new boss, with whom he will start campaigning on Friday in the swing states of Pennsylvania, Ohio and Michigan, and his old friend McCain.

"The choice in this election is clear. These times require more than a good soldier," he said, referring to McCain's military service that includes his legendary five and half years as a prisoner of war in Vietnam. "They require a wise leader who can deliver change. The change everybody knows we need. Barack Obama will deliver that change."

No sooner had word spread of Obama's selection than McCain's campaign unleashed its first of many attacks, citing Biden's earlier statements criticizing Obama for his lack of experience. Biden said last year that Obama was not ready to serve as president, a position that was reversed on Wednesday by both the vice presidential nominee and Bill Clinton.

McCain's campaign has also been ribbing Obama for choosing such a grand setting for his nomination acceptance speech. Invesco Field, home of the Denver Broncos, has been transformed for the Thursday address, and the McCain campaign has dubbed it "The Temple of Obama."

Democrats say they chose the venue so members of the public can witness the historic event.