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.