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 bid to become the nation's first black president.

"America, now is not the time for small plans," the 47-year-old Illinois senator told an estimated 84,000 people packed into Invesco Field, a 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" on health care, education, the economy and more, he said of his Republican rival.

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.

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

Obama delivered his 44-minute nominating 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 running mate, Sen. Joseph Biden of Delaware, leave their convention city today for Pennsylvania, first stop on an eight-week sprint to Election Day.

Rep. John Lewis of Georgia spoke first 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 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 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.

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."

His 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.

"Senator 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?"

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 that 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."