DENVER -- Sen. Barack Obama accepted his historic Democratic presidential nomination with soaring oratory that sought to identify his biography with the American dream and his campaign with a populist call to mend the American economy.

"America, we cannot turn back," Mr. Obama said at the end of his prime-time speech at Denver's Invesco Field, "Not with so much work to be done.... At this moment, in this election, we must pledge once more to march into the future."

As waves of cheers broke over a crowd of nearly 80,000, Mr. Obama's speech aimed to bind his life with the lives of struggling voters. At the same time, he and other Democratic speakers joined in portraying his Republican opponent, Sen. John McCain, as the unquestioning heir to the policies of an unpopular administration.

"Four years ago, I stood before you and told you my story -- of the brief union between a young man from Kenya and a young woman from Kansas who weren't well-off or well-known, but shared a belief that in America, their son could achieve whatever he put his mind to," he said, recalling the electrifying convention speech that made him a national figure even before he was elected to the U.S. Senate.

"It is that promise that has always set this country apart -- that through hard work and sacrifice, each of us can pursue our individual dreams but still come together as one American family, to ensure that the next generation can pursue their dreams as well."

After a summer of partisan debate on national security, Mr. Obama focused on the economy, arguing that Republican policies had failed to respond to crises in housing, health care and credit markets.

"America, we are better than these last eight years," he said.

At times the stadium almost shook with the foot-stomping, roaring appreciation of the partisans who'd copped the toughest ticket in Denver.

"Tonight, more Americans are out of work and more are working harder for less," he said at another point. "More of you have lost your homes and more are watching your home values plummet. More of you have cars you can't afford to drive, credit card bills you can't afford to pay and tuition that is beyond your reach. These challenges are not all of government's making. But the failure to respond is a direct result of a broken politics in Washington and the failed presidency of George W. Bush."

With his speech, Mr. Obama finally took the full spotlight after a convention in which his unprecedented candidacy had competed for attention with Sen. Hillary Rodham Clinton, the woman he had defeated in the longest and most expensive nomination fight in American political history.

Speaking earlier in the week, his wife, Michelle, called his march to the nomination an improbable story. Few would accuse her of hyperbole. Mr. Obama claimed his party's standard for the race against Mr. McCain after being raised in Hawaii and Indonesia by a single mother. He overcame much of his party's political establishment as he vanquished the wife of the last president of his party with a campaign that seized a message of hope and exploited the new tools of digital communication.

Responding to Republican characterizations of him as an elitist, he described a commonplace American background.

"In the face of these young veterans who come home from Iraq and Afghanistan, I see my grandfather who signed up after Pearl Harbor, marched in Patton's army and was rewarded by a grateful nation with the chance to go to college on the GI bill.

"In the face of the young student who sleeps just three hours before working the night shift," he continued, "I think about my Mom, who raised my sister and me on her own while she worked and earned her college degree -- who once turned to Food Stamps but was still able to send us to the best schools in the country."

Alongside the uplifting notes, however, Mr. Obama's words included a flurry of criticisms of his opponent.

"Now I don't believe that Sen. McCain doesn't care what's going on in the lives of Americans. I just think he doesn't know," he said.

Referring to Mr. McCain's observation at their recent twin appearances with the Rev. Rick Warren in California, he added, "Why else would he define middle class as someone making under five million dollars a year?"

"It's not because John McCain doesn't care; it's because John McCain doesn't get it."

In another answer to Republican attacks, he put a harder edge on the post-partisan theme that resonated so strongly in his convention address four years ago.

"The men and women who serve in our battlefields may be Democrats and Republicans and Independents, but they have fought together and bled together and some have died together under the same proud flag," he said sternly. "They have not served a Red America or a Blue America -- they have served the United States of America. So I've got news for you, John McCain. We all put our country first."

The scheduling evoked political history. The first African-American nominee of a major party spoke on the 45th anniversary of Martin Luther King's "I Have a Dream speech."

And the outdoor setting, and even the Greek columns -- the object of immediate GOP ridicule as the "Temple of Obama" -- were reminiscent of the Los Angeles Coliseum address where President John F. Kennedy gave his acceptance speech in 1960.

Most prominent among a parade of speakers that preceded the nominee in the five-hour program was former Vice President Al Gore. The Nobel Prize winner alluded to his own bitter election experience to underline the stakes in November.

"Today we face essentially the same choice we faced in 2000, though it may be even more obvious now, because John McCain, a man who earned our respect on many levels, is now openly endorsing the policies of the Bush-Cheney White House and promising to actually continue them.

"The same policies all over again?" the environmental activist demanded. "Hey, I believe in recycling, but that's ridiculous."

Recognizing the power of the event, the GOP campaign took the unusual step of producing a commercial featuring Mr. McCain congratulating Mr. Obama on his nomination that was to be broadcast in battleground states across the country.

The close of the speech was heralded with a ring of fireworks surrounding the stadium as first Mr. Obama's family and then his new running mate, Sen. Joseph R. Biden Jr., joined the nominee on stage as the crowd's cheers competed with the fireworks.