DENVER - Surrounded by an enormous, adoring crowd, Barack Obama promised a clean break from the "broken politics in Washington and the failed policies of George W. Bush" Thursday night as he embarked on the final lap of his audacious bid to become the nation's first black president.

"America, now is not the time for small plans," the 47-year-old Democratic Illinois senator told an estimated 84,000 people packed into Invesco Field, a huge football stadium at the base of the Rocky Mountains.

He vowed to cut taxes for nearly all working-class families, end the war in Iraq and break America's dependence on Mideast oil within a decade. By contrast, he said, "John McCain has voted with President Bush 90 percent of the time," a scathing indictment of his Republican rival - on health care, education, the economy and more.

Polls indicate a close race between Obama and McCain, the Arizona senator who stands between him and a place in history. On a night 45 years after Martin Luther King Jr. delivered his "I have a Dream Speech," Obama made no overt mention of his own race.

"I realize that I am not the likeliest candidate for this office. I don't fit the typical pedigree" of a presidential candidate was as close as he came to the long-smoldering issue that may well determine the outcome of the election.

Fireworks lit the night sky as Obama, his speech concluded, accepted the cheers of supporters. His wife, Michelle, and their daughters Malia and Sasha joined him as the country music anthem "Only in America" filled the stadium. Vice presidential running mate Joseph Biden and his wife, Jill, joined them onstage.

Depicted by McCain as too young and inexperienced to sit in the Oval Office, Obama responded with an oblique reference to his rival's temper.

"If John McCain wants to have a debate about who has the temperament, and judgment, to serve as the next commander in chief, that's a debate I'm ready to have," he said.

Campaigning as an advocate of a new kind of politics, he suggested at least some common ground was possible on abortion, gun control, immigration and gay marriage.

Obama delivered his 44-minute nomination acceptance speech in an unrivaled convention setting, before a crowd of unrivaled size - the filled stadium, the camera flashes in the night, the made-for-television backdrop that suggested the White House, and the thousands of convention delegates seated around the podium in an enormous semicircle.

Obama and his fellow senator, Biden of Delaware, leave their convention city on Friday for Pennsylvania, first stop on an eight-week sprint to Election Day.

McCain countered the stadium extravaganza with a bold move of his own, hoping to steal some of the political spotlight by spreading word that he had settled on a vice presidential running mate. Minnesota Gov. Tim Pawlenty, former Pennsylvania Gov. Tom Ridge, former Massachusetts Gov. Mitt Romney and Connecticut Sen. Joe Lieberman were in the running.

Rep. John Lewis of Georgia spoke from the convention stage of the anniversary of King's memorable speech.

"Tonight we are gathered here in this magnificent stadium in Denver because we still have a dream," said the Georgia lawmaker, who marched with King, supported Obama's primary rival, Hillary Rodham Clinton, then switched under pressure from younger black leaders in his home state and elsewhere.

Obama's aides were interested in a different historical parallel from King - Obama was the first to deliver an outdoor convention acceptance speech since John F. Kennedy did so at the Los Angeles Coliseum in 1960.

In his speech, Obama pledged to jettison Bush's economic policy - and replace it with his own designed to help hard-pressed families.

"I will cut taxes for 95 percent of all working families. Because in an economy like this, the last thing we should do is raise taxes on the middle class," he said.

The speech didn't mention it, but Obama has called for raising taxes on upper-income Americans to help pay for expanded health care and other domestic programs.

He did not say precisely what he meant by breaking the country's dependence on Mideast oil, only that Washington has been talking about doing it for 30 years "and John McCain has been there for 26 of them."

Criticized by the GOP for his thin foreign policy portfolio, Obama said he welcomed a national security debate with McCain.

"We are the party of Roosevelt. We are the party of Kennedy. So don't tell me that Democrats won't defend this country," Obama said. "I will never hesitate to defend this nation."

He said McCain had no standing on foreign policy, not after backing the Iraq war from the start and rejecting timetables for withdrawal now accepted by Bush. "John McCain stands alone in his stubborn refusal to end a misguided war," he said.

Obama's pledge to end the war in Iraq responsibly was straight from his daily campaign speeches.

"I will rebuild our military to meet future conflicts. But I will also renew the tough, direct diplomacy that can prevent Iran from obtaining nuclear weapons," he added.

As he does so often while campaigning, Obama also paid tribute to McCain's heroism - the 72-year-old Arizona senator was a prisoner of war in Vietnam - then assailed him.

"Sen. McCain likes to talk about judgment, but really, what does it say about your judgment when you think George Bush was right more than 90 percent of the time?

Former Vice President Al Gore picked up on the same theme. "If you like the Bush-Cheney approach, John McCain's your man. If you want change, then vote for Barack Obama and Joe Biden," he declared.

The much-discussed stage built for the program was evocative of the West Wing at the White House, with 24 American flags serving as a backdrop. A blue carpeted runway jutted out toward the infield, and convention delegates ringed the podium. Thousands more sat in stands around the rim of the field.

The wrap-up to the party convention blended old-fashioned speechmaking, Hollywood-quality stagecraft and innovative, Internet age politics.

The list of entertainers ran to Sheryl Crow, Stevie Wonder and will.i.am, whose Web video built around Obama's "Yes, we can" rallying cry quickly went viral during last winter's primaries.

In a novel bid to extend the convention's reach, Obama's campaign decided to turn tens of thousands of partisans in the stands into instant political organizers.

They were encouraged to use their cell phones to send text messages to friends as well as to call thousands of unregistered voters from lists developed by the campaign.

In all, Obama's high command said it had identified 55 million unregistered voters across the country, about 8.1 million of them black, about 8 million Hispanic and 7.5 million between the ages of 18 and 24.

Those are key target groups for Obama as he bids to break into the all-white line of U.S. presidents and at the same time restore Democrats to the White House for the first time in eight years.

The Democratic man of the hour paid a brief visit to members of his home-state Illinois delegation before the curtain went up on his show. "I came by (because) I had this speech tonight. I wanted to practice it out on you guys. See if it worked on a friendly audience," he joked.

There was no joking about the stakes in the speech, a once-in-a-campaign opportunity to speak to millions of voters who have yet to make up their minds between McCain and him. The polls show a close race nationally, with more than enough battleground states tight enough to tip the election either way.

Obama's hopes of victory rely on holding onto the large Democratic base states such as California, New York, Michigan and his own Illinois, while eating into territory that voted for George W. Bush. Ohio tops that list, and Democrats have also targeted Montana, North Dakota, Virginia and New Mexico, among others, as they try to expand their Electoral College map.

McCain was in Ohio as Obama spoke, and after a series of sharply negative convention week television commercials, his campaign aired a one-night advertisement that complimented Obama and noted the speech occurred on the anniversary of King's famous address.

"Senator Obama, this is truly a good day for America. Too often the achievements of our opponents go unnoticed. So I wanted to stop and say, 'Congratulations,'" McCain says in the ad.

"How perfect that your nomination would come on this historic day. Tomorrow, we'll be back at it. But tonight Senator, job well done."

DENVER - Barack Obama cast his presidential nomination as proof that no dreams are too high, savoring a historic moment for himself and the nation Thursday before setting out on a difficult struggle to break another barrier for a black American.

Obama's success in obtaining the Democratic nomination was indeed a remarkable achievement, reached despite the misgivings of some Americans uncomfortable with electing the son of an African immigrant - not "the typical pedigree," as he put it.

He used his acceptance speech at the Democratic National Convention in part to allay those concerns, to show Americans that he is one of them - not born of wealth or privilege, his gains made of hard work and sacrifice.

"This moment - this election - is our chance to keep, in the 21st Century, the American promise alive," Obama said. He put himself in the shadow of great leaders like John Kennedy, Franklin Roosevelt and Martin Luther King, Jr., as well as his humble parents.

His speech was the culminating moment of the Democrats' four-day convention, the launching point for a difficult fall campaign against McCain.

The stakes could not have been higher - for the future of this campaign and the past of racial politics. It came on the 45th anniversary of one of the greatest speeches in American history, King's "I Have a Dream" address.

An enthusiastic crowd of 84,000 - unprecedented for a political convention - literally shook the stadium at Invesco Field at Mile High with their stomping feet, every participant equipped by organizers with an American flag. More important was the audience of millions of Americans watching on television, a tougher crowd, as Obama spoke before a backdrop of columns reminiscent of the White House portico.

Looking for validation, Obama gave unknown Americans from battleground states prime-time speaking roles to explain their struggles and how the candidate could help them. And Obama himself highlighted the stories of working class Americans, the kinds of voters who have expressed wariness of his candidacy - the woman about to retire in Ohio worried about health care costs, the Indiana worker who lost his job to competition from China, the veterans living on the streets or in poverty, the military families in the midst of repeat tours of duty.

He wanted them to know he was one of them. He said he sees his World War II veteran grandfather in the faces of veterans returning from Iraq and Afghanistan, recognizes his mother in the overworked student yearning to give her children a better life and hears his grandmother in the voice of the businesswoman facing workplace discrimination.

"I get it," Obama said. "I realize that I am not the likeliest candidate for this office. I don't fit the typical pedigree, and I haven't spent my career in the halls of Washington. But I stand before you tonight because all across America something is stirring. What the nay-sayers don't understand is that this election has never been about me. It's been about you."

For those voters with another concern - that a first-term senator who just turned 47 isn't experienced enough to lead the country - Obama had an answer, too, in a list of policy proposals that he argued would improve their lives. He promised tax cuts that would benefit workers, an end to dependence on Middle East oil, more funding for education, health care for every American and an end to the war in Iraq.

"America, now is not the time for small plans," Obama said.

And he tried to raise concerns about his rival, Republican John McCain, by saying he's too much like the unpopular President Bush.

"John McCain has voted with George Bush 90 percent of the time," Obama said. "I don't know about you, but I'm not ready to take a 10 percent chance on change."

With the nomination in hand, Obama could afford to pause - if only for a moment - to reflect on the path that took him from untested rising star at the Democratic convention just four years ago to the party's standard-bearer this time and a symbol of hope to millions of Americans yearning for change.

Obama himself took note of the transformation.

"Four years ago, I stood before you and told you my story - of the brief union between a young man from Kenya and a young woman from Kansas who weren't well-off or well-known - but shared a belief that in America, their son could achieve whatever he put his mind to."

Then he launched himself into the task at hand, persuading voters that he is the leader for "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 didn't flinch from offering himself as ready not only for the title of president but also of "commander in chief."

"If John McCain wants to have a debate about who has the temperament, and judgment, to serve as the next commander in chief, that's a debate I'm ready to have," Obama said.