Delegates who packed into their convention hall were serenaded by singer James Taylor and rocked by R&B blues artist Mary J. Blige as they awaited Obama's speech.
There was no end to the jabs aimed at Romney and the Republicans.
"Ask Osama bin Laden if he's better off than four years ago," said Massachusetts Sen. John Kerry, who lost the 2004 election in a close contest with President George W. Bush. It was a mocking answer to the Republicans' repeated question of whether Americans are better off than when Obama took office.
The campaign focus was shifting quickly â€" to politically sensitive monthly unemployment figures due out Friday morning and the first presidential debate on Oct. 3 in Denver. Wall Street hit a four-year high a few hours before Obama's speech after the European Central Bank laid out a concrete plan to support the region's struggling countries.
The economy is by far the dominant issue in the campaign, and the differences between Obama and his challenger could hardly be more pronounced.
Romney wants to extend all tax cuts that are due to expire on Dec. 31 with an additional 20 percent reduction in rates across the board, arguing that job growth would result. He also favors deep cuts in domestic programs ranging from education to parks, repeal of the health care legislation that Obama pushed through Congress and landmark changes in Medicare, the program that provides health care to seniors.
Obama wants to renew the tax cuts except on incomes higher than $250,000, saying that millionaires should contribute to an overall attack on federal deficits. He also criticizes the spending cuts Romney advocates, saying they would fall unfairly on the poor, lower-income college students and others. He argues that Republicans would "end Medicare as we know it" and saddle seniors with ever-rising costs.
After two weeks of back-to-back conventions, the impact on the race remained to be determined.
You're not going to see big bounces in this election," said David Plouffe, a senior White House adviser. "For the next 61 days, it's going to remain tight as a tick."
Romney wrapped up several days of debate rehearsals with close aides in Vermont and is expected to resume full-time campaigning in the next day or two.
In a brief stop to talk with veterans on Thursday, he defended his decision to omit mention of the war in Afghanistan when he delivered his acceptance speech last week at the Republican National Convention. He noted he had spoken to the American Legion only one day before.
Romney's campaign released its first new television ad since the convention season began.
It shows Clinton sharply questioning Obama's credibility on the Iraq War in 2008, saying "Give me a break, this whole thing is the biggest fairy tale I've ever seen." Obama was running against Hillary Rodham Clinton at the time for the Democratic nomination.
It will likely be a week or more before the two campaigns can fully digest post-convention polls and adjust their strategies for the fall.
Based on the volume of campaign appearances to date and the hundreds of millions of dollars spent already on television advertising, the election appears likely to be decided in a small number of battleground states. The list includes New Hampshire, Virginia, Ohio, Colorado, Nevada and Iowa, as well as Florida and North Carolina, the states where first Republicans and then Democrats held their conventions. Those states hold 100 electoral votes among them, out of 270 needed to win the White House.
Money has become an ever-present concern for the Democrats, an irony given the overwhelming advantage Obama held over John McCain in the 2008 campaign.
This time, Romney is outpacing him, and independent groups seeking the Republican's election are pouring tens of millions of dollars into television advertising, far exceeding what Obama's supporters can afford.
Associated Press writers Leo Buckle, Ben Feller, Ken Thomas, Matt Michaels and Jim Kuhnhenn in Charlotte, Calvin Woodward, Jennifer Agiesta, Jack Gillum and Josh Lederman in Washington, Kasie Hunt in Vermont and Thomas Beaumont and Steve Peoples in Iowa contributed to this report
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