Speaking to a nation fighting two wars, struggling with a weakened economy and growing doubtful about the future, Obama said he would make the fall campaign a choice between a continuation of eight years of Republican policies and a new direction aimed at ending the conflict in Iraq and easing the economic insecurities of working families.

"These challenges are not all of government's making," he added. "But the failure to respond is a direct result of a broken politics in Washington and the failed policies of George W. Bush. . . . I say to the American people, to Democrats and Republicans and independents across this great land: Enough!"

Criticism of McCain was the thread woven throughout the speech. For the past month, Obama has been under attack from his rival and the Republicans. On Thursday night, in perhaps the most important speech of his political career, he answered back.

McCain has charged that Obama is not experienced enough to protect the country. "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 declared.

McCain and the Republicans have mocked him as an empty-suited celebrity enamored with the sound of his own voice, a haughty elitist who cares little about average Americans. In response, Obama cited the lives of his mother, who used food stamps at one point; his grandmother, who rose from being a secretary to middle management; and his grandfather, who fought in Gen. George S. Patton's Army.

"I don't know what kind of lives John McCain thinks that celebrities lead, but this has been mine," he said. "These are my heroes. Theirs are the stories that shaped me. And it is on their behalf that I intend to win this election and keep our promise alive as president of the United States."

To those who have questioned his patriotism, he sounded a call for a turn away from the partisanship that has marked politics for a decade or more -- and challenged his rivals to make this election about big and bold issues, not small and petty arguments.

"The times are too serious, the stakes are too high for this same partisan playbook. So let us agree that patriotism has no party. I love this country, and so do you, and so does John McCain."

And then in a reprise of one of the most remembered lines of his convention keynote address at the Democratic convention four years ago in Boston, he said the men and women who have fought and died for this country may have been of different parties but all died under the same flag.

"They have not served a red America or a blue America -- they have served the United States of America," he said. "So I've got news for you, John McCain: We all put our country first."

He also challenged McCain not to stoop to questioning his motives. "What I will not do is suggest that the senator takes his positions for political purposes," he said. "Because one of the things that we have to change in our politics is the idea that people cannot disagree without challenging each other's character and patriotism."

The crowd that gathered for Obama's speech, estimated at more than 84,000, exceeded the audience assembled for John F. Kennedy's acceptance speech in 1960 in Los Angeles. As tens of thousands of flag-waving people streamed into the Denver Broncos' vast football stadium, the scene threatened to diminish the words in Obama's text.

But his real audience was not the crowd that waited hours to get into Invesco Field at Mile High but rather the voters who are trying to take a measure of a still relatively unknown politician seeking to lead the United States at a critical moment in history.

His checklist, sketched out by Democratic elected officials and delegates during the first three days of the convention, was long. In fact, it was almost more than any single speech could accomplish -- a combination of poetry and prose that would put to rest questions about his candidacy, connect with undecided voters and hit the kind of emotional high notes that long have thrilled his followers.

Some Democrats said his highest priority should be mending any last rifts with supporters of Sen. Hillary Rodham Clinton, his rival for the nomination, to ensure that Democrats leave Denver united. Other party members noted that Obama needed to fill in the blank spots in his own biography for voters who wonder who he is, where he came from and how his unusual life ties directly to theirs.

The portrait he painted of McCain was that of a man who had served his country nobly but who is out of touch with struggling families and is joined at the hip with President Bush on foreign and domestic policy.

"John McCain has voted with George Bush 90 percent of the time," he said. "Senator McCain likes to talk about judgment, but really, what does it say about your judgment when you think George Bush has been right more than 90 percent of the time? I don't know about you, but I'm not ready to take a 10 percent chance on change."

Another section of the speech dealt with criticism that Obama has pitched his call for change at such a lofty level that he has left many Americans wondering what he would actually do as president to change their lives. "Let me spell out exactly what that change would mean if I am president," he said.

There followed a lengthy list of policy prescriptions. Tax cuts for 95 percent of working families, a pledge to end dependence on foreign oil in 10 years through investment in natural gas, safe nuclear power, clean coal technology, more fuel-efficient cars and $150 billion invested in alternative energy.

He promised to improve education, pay teachers more, provide every American with affordable health care and protect Social Security's financial future.

The speech did not, however, set the clear priorities that some of his critics say his governing agenda has lacked. Whether that will come over the next 60 days, as the campaign is fought out, is doubtful.

For a politician who won the nomination in part through the power of his rhetoric, who probably would not have been a candidate this year were it not for his electrifying address at the convention four years ago, Obama was under considerable pressure Thursday night to deliver a speech of special force and power. What he gave here was a combination of old and new -- new toughness coupled with the message that got him to this point.

"It's time for them to own their failure," he said of Republicans. "It's time for us to change America."

Obama's challenge between now and Election Day is to make that stick.