DENVER — Before an enormous, adoring crowd, Senator Obama promised a clean break from the “broken politics in Washington and the failed policies of George W. Bush” tonight 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. Assailing his Republican rival, he said it isn’t that Senator McCain doesn’t care about the economic problems of working people — “he just doesn’t get it.”

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.

Mr. Obama delivered his 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.