The poignancy of that breakthrough wasn’t lost on the crowd or the candidate.
Framing the fight with rival John McCain, Mr. Obama pulled few punches, vowing to seek common ground on immigration, energy, gay marriage, abortion and other vexing issues while painting the Republican as tough-talking but out-of-touch, likable but misguided, a maverick in thrall of his party’s right wing.
“I’ve got news for you, John McCain. We all put our country first,” he said.
Republicans said the speech only proved that Mr. Obama remains unready to lead. But Democrats marveled at the 47-year-old senator’s ability to weave a lofty vision, withering attacks, and fresh details of what his promise of “change” would mean.
“It’s time for us to change America,” Mr. Obama said – one of 15 times he invoked the term.
The event was filled with historic echoes. It came on the 45 anniversary of Martin Luther King’s “I Have a Dream” speech, and Mr. Obama repeatedly invoked the legacy of that “preacher from Georgia.”
Even the venue was laden with meaning. No candidate since John Kennedy had picked an outdoor stadium to accept his nomination. And in a clear echo of Kennedy’s goal of a manned moon shot within a decade, Mr. Obama’s called for ambitious thinking once again, proposing for instance a goal to end dependence on foreign oil in a decade.
The weather in Denver was picture perfect, the Rockies providing a backdrop to Invesco Field at Mile High, where the Broncos play football, the harsh sun fading to a crisp mountain evening by the time Mr. Obama took the stage.
The gathering combined elements of music festival, state fair and sporting event — but with sharpshooters pacing the top of the scoreboard.
Delegates from Texas and other states had seats on field level, around a deep blue stage with white columns evoking those at the White House. Tens of thousands of supporters, many from Colorado, lined up for hours to get through security. Inside, they endured long lines for overpriced hot dogs and popcorn, and jumped to their feet in waves during lulls in the action.
“It’s astonishing. He pulled out all the young people. He’s united everyone else behind him. Look at the faces,” said Rogelio “Roy” Flores of San Antonio, national vice president for the American Federation of Government Employees, who told his 20-something colleagues to take it all in, because they’ll never see anything like it again.
Mr. Obama used the spotlight to denounced the “Bush-McCain foreign policy” that has squandered international goodwill and undermined long-term American interests in the Middle East and Eastern Europe.
“If John McCain wants to follow George Bush with more tough talk and bad strategy, that is his choice – but it is not the change we need,” Mr. Obama said, and “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.”
Mr. Obama also blasted Mr. McCain as out of touch with ordinary Americans’ economic struggles, invoking the quip that got Phil Gramm ousted as an official McCain adviser.
“A nation of whiners?” Mr. Obama said, drawing boos from the crowd without naming the Texas senator behind the assessment. “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. Why else would he define middle class as someone making under $5 million a year?”
McCain campaign spokesman Tucker Bounds called Mr. Obama’s 42-minute speech full of misleading statements and said it showed that “Barack Obama is still not ready to be president.”
Expectations were mile-high by the time Mr. Obama took the stage.
Democrats hunger to regain the White House, but Mr. Obama has slipped in the polls in recent weeks.
And there was Mr. Obama’s reputation as an oratorical master. His keynote address at the 2004 convention catapulted him from obscure Senate candidate to up-and-coming sensation. But Republicans have used his ability to draw huge crowds and mesmerize them to accuse him of being more style than substance.
Backers of his Democratic primary rival Hillary Rodham Clinton arrived seeking some recognition, and he made that an early order of business.
“We did our part. Now it’s his turn,” said Garry Mauro, who chaired the Clinton campaign in Texas and wore a “Hillary Supporters for Obama” button to Invesco.
He also thought the time had come for Mr. Obama to rebut the rap that he is more style than substance.
“There’s going to be 25 million Americans watching,” Mr. Mauro said. “It’s 10 weeks before the election. Now’s the time to get a little more specific.”
Mr. Obama’s warm-up acts ranged from performers Sheryl Crow, will.i.am and Stevie Wonder to also-rans for president and vice president.
Former Vice President Al Gore pressed the case that Mr. McCain embraces Bush-Cheney policies with a quip that played off his environmentalism: “Hey, I believe in recycling, but that’s ridiculous.”