U.S. Democrats ready celebration for Obama

"We want to open up the convention to make sure that everybody who wants to come can join in the party," said Obama, 47, who appeared on stage after the acceptance speech of his newly minted running mate, Delaware Sen. Joe Biden.

National conventions are often the first time voters start to pay attention to a presidential race. Opinion polls show many voters are still unfamiliar with Obama and concerned about his readiness for the job.

Speakers at the convention have tried to address those concerns, led by rousing testimonials for Obama from former rival Hillary Clinton, her husband former President Bill Clinton and Biden.

"Barack Obama is ready to lead America and to restore American leadership in the world," Bill Clinton told flag-waving Democrats, answering a frequent line of Republican attack.

Obama is running even with McCain in opinion polls. Back-to-back nominating conventions -- the Republicans go next week -- will give voters a chance to compare and contrast.

Obama’s senior strategist, David Axelrod, told reporters the speech would focus on Obama’s vision for the country’s future.

THE CASE FOR CHANGE

"He’s going to lay out a case for change. He’s going to set the stakes of this election, the risks of continuing down the road we’re on which is plainly what Senator McCain is offering," Axelrod said.

"And he’s going to talk about an alternative path that’s rooted in the best of what this country is and the kind of future we can build if we take it," he said.

Some Democrats have said Obama needs to be more specific about his priorities as president, and draw a sharp contrast with McCain. Axelrod said both elements would be included.

"His goal is to talk to the American people directly about the challenges that we face and what it’s going to take to solve them," he said.

Obama hopes to get a lift in the polls from his choice of Biden for vice president and from the convention, but McCain will try to reclaim the momentum on Friday with an announcement of his running mate.

McCain has decided on his choice, a Republican official said, and will announce it at a rally in Ohio. Former Massachusetts Gov. Mitt Romney and Minnesota Gov. Tim Pawlenty are among the possible candidates.

If elected McCain would be the oldest first term president to take office. His 72nd birthday is on Friday.

Obama was formally nominated on Wednesday in an emotional show of unity after Hillary Clinton, his vanquished rival, appeared on the convention floor to ask Democrats to suspend their roll call of the states and make Obama the nominee by acclamation.

"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," she said to raucous cheers.

Former Vice President Al Gore will speak to the convention before Obama on Thursday. New Mexico Gov. Bill Richardson, a Hispanic, will also make an appearance. Hispanics are a fast growing segment of the U.S. electorate and a potentially vital voting bloc.

The last presidential candidate to accept the nomination in an open-air football stadium was John Kennedy, who spoke to the Democratic convention at the Los Angeles Coliseum before 80,000 supporters in 1960.

(Additional reporting by Steve Holland and Caren Bohan; Editing by Howard Goller and David Wiessler)