His re-election in doubt, President Barack Obama accepted his party's presidential nomination Thursday night, acknowledging slow progress in solving the nation's economic woes but declaring, "Our problems can be solved, our challenges can be met."

"The path we offer may be harder, but it leads to a better place," he said.

"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.

First Lady Michelle Obama and the couple's daughters, Malia and Sasha, joined the president on stage in the moments after the speech, followed by other family members and Vice President Joe Biden and his wife, Jill. Strains of "Only in America" filled the hall as confetti filled the air.

His speech was the final act of his national convention, and the opening salvo of a two-month drive toward Election Day in his race against Republican rival Mitt Romney. The contest is close for the White House in a dreary season of economic struggle for millions.

With unemployment at 8.3 percent, Obama 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 the Great Depression 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."

Romney answered the speech by issuing a statement saying Obama hadn't kept his promises: “Tonight President Obama laid out the choice in this election, making the case for more of the same policies that haven't worked for the past four years."

The convention's final night also included a nomination acceptance speech from Biden, whose appeal to blue collar voters rivals or even exceeds Obama's own.

Biden told the convention that he had watched as Obama "made one gutsy decision after another" to stop an economic free-fall after they took office in 2009.

Now, he said, "we're on a mission to move this nation forward — from doubt and downturn to promise and prosperity."

With Obama in the hall listening, Biden jabbed at the president's challenger, as well.

"I found it fascinating last week — when Governor Romney said that as president he'd take a jobs tour,” he said. “Well with all his support for outsourcing — it's going to have to be a foreign trip."

Mrs. Obama, popular with the public, introduced her husband, two nights after she delivered her own speech in the convention's opening session.

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.