Making history as the first black presidential nominee of a major party, Barack Obama accepted the honor Thursday at the Democratic National Convention by asking voters to invigorate the "American promise" and pointing to his own remarkable ascent as proof that promise is still alive.

Addressing a massive crowd of more than 85,000 cheering supporters at Invesco Field at Mile High in Denver, the first-term Illinois senator marked his nomination on the 45th anniversary of Martin Luther King Jr.'s "I Have a Dream" speech.

"With profound gratitude and great humility, I accept your nomination for the presidency of the United States," he said at the top of the speech.

In the high-stakes address that was at times policy-oriented but frequently critical of the last eight years, Obama pointedly accused Republican rival John McCain of representing a stagnant and tired political philosophy that would mirror the Bush administration.

But he also took time to acknowledge the historic heft of the moment.

"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," Obama said.

The speech came just four years after he delivered the keynote address at the 2004 Democratic convention that launched him on his insurgent campaign for the White House.

"It is that promise that's 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. That's why I stand here tonight," he continued.

Obama, who has been cast as aloof and elitist by McCain, used his address to cast his rival as the candidate who is out of touch with the concerns of ordinary Americans.

Saying "now is not the time for small plans," he declared voters are converging on a "defining" moment, when the "American promise has been threatened once more."

"This moment, this election, is our chance to keep, in the 21st century, the American promise alive. Because next week, in Minnesota, the same party that brought you two terms of George Bush and Dick Cheney will ask this country for a third," 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," he said.

"Now, I don't believe that Senator McCain doesn't care what's going on in the lives of Americans. I just think he doesn't know. ... It's because John McCain doesn't get it," he said. "For over two decades, he's subscribed to that old, discredited Republican philosophy -- give more and more to those with the most and hope that prosperity trickles down to everyone else."

The McCain campaign quickly responded to the address by saying Obama had distorted the country about his own record.

"When the temple comes down, the fireworks end and the words are over, the facts remain: Senator Obama still has no record of bipartisanship, still opposes offshore drilling, still voted to raise taxes on those making just $42,000 per year, and still voted against funds for American troops in harm's way. The fact remains: Barack Obama is still not ready to be president," said McCain spokesman Tucker Bounds.

Obama also lashed out at the administration's foreign policy while addressing the needs of Americans suffering in an economic downturn.

“We measure the strength of our economy not by the number of billionaires we have or the profits of the Fortune 500, but by whether someone with a good idea can take a risk and start a business, or whether the waitress who lives on tips can take a day off to look after a sick kid without losing her job -– an economy that honors the dignity of work," Obama said.

Often accused of being too vague and lofty in his rhetoric, Obama spelled out specifics in his speech for how he would improve the lives of American working-class families. He said he will invest $150 billion over the next decade in alternative energies -- wind and solar power and biofuels; end U.S. dependence on foreign oil in 10 years, rebuild the military and renew direct diplomacy aimed at ending Iran's nuclear ambitions; eliminate capital gains taxes on small businesses that create high-tech, high-pay jobs and cut taxes for 95 percent of all working families.

“Change means a tax code that doesn’t reward the lobbyists who wrote it, but the American workers and small businesses who deserve it," he said. “Unlike John McCain, I will stop giving tax breaks to corporations that ship our jobs overseas, and I will start giving them to companies that create good jobs right here in America."

Obama's running mate Joe Biden made a surprise appearance about an hour before Obama took the stage, pledging to make the Democratic ticket work for the working class.

"When we talked about an open convention, this is what Democrats meant," he said.

Obama delivered the address in front of two sets of Greek-inspired columns and a façade resembling a Washington, D.C., building. Delegates lined the floor of the stadium, while tens of thousands of spectators filled the bleachers to the top.

The Republican war room monitoring the convention offered up several barbs to make fun of the production, which also included high-end celebrity performances from musicians like will.i.am and Sheryl Crow.

But on Thursday, McCain separated himself from the attacks to offer his congratulations to Obama.

"How perfect that your nomination would come on this historic day," he said in a one-night-only TV ad. "Tomorrow, we'll be back at it. But tonight senator, job well done."

The speech was delivered on the final day of the Democratic National Convention and for Obama was his last chance to encapsulate the Democratic message before Republicans head to their convention in St. Paul next week.

The convention was marked by some tensions between his campaign and supporters of vanquished rival Hillary Clinton, but both Hillary and former President Bill Clinton gave full-throated endorsements to Obama earlier in the week.