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 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.

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 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 on Friday for Pennsylvania, first stop on an eight-week sprint to Election Day.

McCain countered 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 canceled all scheduled appearances for the next two days, stoking speculation that he was the one.

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 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.”

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.

“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.

His new running mate, Sen., Joseph Biden of Delaware, was brutally frank about the Democrats’ chances in an appearance before one state’s delegation. “This is not hyperbole: We cannot win without Pennsylvania,” he said.

Polling shows the race for that state’s 21 electoral votes close. Both the two previous Democratic candidates, Al Gore and John Kerry carried Pennsylvania over Bush.

Biden, who was born in Scranton, Pa., and represents a state that shares a border with Pennsylvania, is expected to spend large amounts of time campaigning in the state over the next several weeks.

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.”