"John McCain has voted with George Bush 90 per cent of the time. Senator McCain likes to talk about judgment, but really, what does it say about your judgment when you think George Bush was right more than 90 per cent of the time?
"I don't know about you, but I'm not ready to take a 10 per cent chance on change," the Illinois senator said.
Amid fireworks and confetti, Obama was joined onstage afterwards by his wife, Michelle, and two daughters, along with Joe Biden, his running-mate, and his family.
But McCain immediately hit back following the speech, with his campaign issuing a statement saying Obama was not ready to become president.
"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."
Obama paid tribute to his former rival, Hillary Clinton, and her husband, Bill Clinton, the former president, in a push for party unity early in his speech as the Democrats gear up for the battle for the White House on November 4.
Al Jazeera's Rob Reynolds said the speech contained a level of detail that would satisfy those who have been complaining that Obama's speeches are full of high-flown rhetoric, but lack specifics.
Obama spent a large of his speech addressing what many polls suggest is the greatest concern among voters: the economy.
He said the "economic turmoil" highlighted by soaring home foreclosures, plummeting house values and rising fuel prices was "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," he said.
Marwan Bishara, Al Jazeera's senior political analyst, said the speech was progressive by US standards.
"He made sure that he is in no way mentioned as an African-American. This was an American speaking to other Americans. Colour was not part of this event. It was nuanced," Bishara said.
Details of change
Spelling out what changes he would make as president, Obama promised to "cut taxes for 95% of all working families" and "finally end our dependence on oil from the Middle East" in 10 years.
"Washington has been talking about our oil addiction for the last 30 years, and by the way John McCain has been there for 26 of them.
"And in that time, he's said no to higher fuel-efficiency standards for cars, no to investments in renewable energy, no to renewable fuels. Today, we import triple the amount of oil as the day that Senator McCain took office.
Saying he would tap the country's "natural gas reserves, invest in clean coal technology, and find ways to safely harness nuclear power", Obama also promised to invest $150bn over the next decade in affordable, renewable sources of energy – wind power and solar power and the next generation of biofuels.
But he also said that there needed to be "a renewed sense of responsibility from each of us ... each of us must do our part to make our homes and businesses more efficient".
On defence and security – considered by many to be McCain's strongest policy area – Obama said he was ready to debate McCain on "who has the temperament, and judgment, to serve as the next commander-in-chief".
"For while senator McCain was turning his sights to Iraq just days after 9/11, I stood up and opposed this war, knowing that it would distract us from the real threats we face," he said.
|The Democratic candidate spoke in front of 84,000 supporters [GALLO/GETTY]
"And today, as my call for a time frame to remove our troops from Iraq has been echoed by the Iraqi government and even the Bush administration, even after we learnt that Iraq has a $79bn surplus while we're wallowing in deficits, John McCain stands alone in his stubborn refusal to end a misguided war.
"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 that America needs."
Obama said he would "end this war in Iraq responsibly, and finish the fight against al-Qaeda and the Taliban in Afghanistan".
"But I will also renew the tough, direct diplomacy that can prevent Iran from obtaining nuclear weapons... And I will restore our moral standing so that America is once more the last, best hope for all who are called to the cause of freedom, who long for lives of peace, and who yearn for a better future."