"Four more years," delegates chanted over and over as the 51-year-old Obama stepped to the podium, noticeably grayer than four years ago when he was a history-making candidate for the White House.
The president's speech was the final act of a pair of highly scripted national political conventions in as many weeks, and the opening salvo of a two-month drive toward Election Day that pits Obama against Republican rival Romney. The contest is ever tighter for the White House in a dreary season of economic struggle for millions.
Vice President Joe Biden preceded Obama at the convention podium and proclaimed, "America has turned the corner" after experiencing the worst economic crisis since the Great Depression.
Obama didn't go that far in his own remarks, but he said firmly, "We are not going back, we are moving forward, America."
With unemployment at 8.3 percent, the president said the task of recovering from the economic disaster of 2008 is exceeded in American history only by the challenge Franklin Delano Roosevelt faced when he took office in 1933.
"It will require common effort, shared responsibility and the kind of bold persistent experimentation" that FDR employed, Obama said.
In an appeal to independent voters who might be considering a vote for Romney, he added that those who carry on Roosevelt's legacy "should remember that not every problem can be remedied with another government program or dictate from Washington.
He said, "The truth is, it will take more than a few years for us to solve challenges that have built up over the decades."
In the run-up to Obama's speech, delegates erupted in tumultuous cheers when former Arizona Rep. Gabrielle Giffords, grievously wounded in a 2011 assassination attempt, walked onstage to lead the Pledge of Allegiance. The hall grew louder when she blew kisses to the crowd.
And louder still when huge video screens inside the hall showed the face of Osama bin Laden, the terrorist mastermind killed in a daring raid on his Pakistani hideout by U.S. special operations forces on a mission approved by the current commander in chief.
The hall was filled to capacity long before Obama stepped to the podium, and officials shut off the entrances because of a fear of overcrowding for a speech that the campaign had originally slated for the 74,000-seat football stadium nearby. Aides said weather concerns prompted the move to the convention arena, capacity 15,000 or so.
Obama's campaign said the president would ask the country to rally around a "real achievable plan that will create jobs, expand opportunity and ensure an economy built to last."
He added, "The truth is it will take more than a few years for us to solve challenges that have built up over a decade."
In convention parlance, both Obama and Biden were delivering acceptance speeches before delegates who nominated them for new terms in office.
But the political significance went far beyond that - the moment when the general election campaign begins in earnest even though Obama and Romney have been pointing toward a Nov. 6 showdown for months.