US President Barack Obama on Thursday implored Americans to grant him a second term to complete his battered crusade for change, warning of the starkest election choice in a generation.

Four years after his historic election win, Obama accepted the Democratic Party's nomination for a second time, dispensing hard truths on US economic ills and warning that Republican Mitt Romney would endanger America abroad.

Sketching an agenda to create millions of jobs, cut $4 trillion from the deficit and bolster his nation's strength overseas, Obama refused to abandon the hope of 2008, saying: "know this, America: our problems can be solved."

"When you pick up that ballot to vote -- you will face the clearest choice of any time in a generation," Obama said, forecasting fateful choices looming on jobs and taxes and war and peace.

"The path we offer may be harder, but it leads to a better place and I'm asking you to choose that future," he said, warning Romney would gut the middle class and return to 'blustering and blundering' abroad.

"America, I never said this journey would be easy, and I won't promise that now. Yes, our path is harder -- but it leads to a better place. Yes our road is longer -- but we travel it together. We don't turn back. We leave no one behind."

Obama also blasted Romney and his vice presidential nominee Paul Ryan, as neophytes that would endanger US national security, comparing their thin resume with his own 'tested and proven' leadership.

"My opponent and his running mate are new to foreign policy, but from all that we've seen and heard, they want to take us back to an era of blustering and blundering that cost America so dearly."

"After all, you don't call Russia our number one enemy -- and not Al-Qaeda -- unless you're still stuck in a Cold War time warp.

"You might not be ready for diplomacy with Beijing if you can't visit the Olympics without insulting our closest ally.''

"My opponent said it was 'tragic' to end the war in Iraq, and he won't tell us how he'll end the war in Afghanistan. I have, and I will."

Obama cast his speech as a rallying call for Americans to unite to tear open the gridlock that has paralyzed Washington, warning Romney would fire teachers, impoverish students, all to give more tax breaks to millionaires.

"We've been there, we've tried that, and we're not going back. We're moving forward," Obama said, drawing lusty cheers from 15,000 people packed into a sports arena at the Democratic National Convention in North Carolina.

In many ways, Obama's address was an attempt to reconcile the heady promises of hope and change he dispensed four years ago, with the reality of impossible to meet expectations and an economy still staggering from the Great Recession.

"If you turn away now -- if you buy into the cynicism that the change we fought for isn't possible -- well, change will not happen.

"If you give up on the idea that your voice can make a difference, then other voices will fill the void: lobbyists and special interests; the people with the $10 million checks who are trying to buy this election.

"Washington politicians who want to decide who you can marry, or control health care choices that women should make for themselves."

Obama also posed as a teller of hard truths, arguing that recovery was bound to be hard from the worst recession in decades.

"You didn't elect me to tell you what you wanted to hear. You elected me to tell you the truth," Obama said.

"And the truth is, it will take more than a few years for us to solve challenges that have built up over decades.

"It will require common effort, shared responsibility, and the kind of bold, persistent experimentation that Franklin Roosevelt pursued during the only crisis worse than this one."

Democrat Roosevelt took power in 1933, amid a banking crisis that threatened American capitalism itself, and steered the country to recovery and through the flames of World War II.

Obama took the convention stage knowing that history suggests a sickly economy often dooms an incumbent president seeking re-election.

"On every issue, the choice you face won't be just between two candidates or two parties," Obama said.

"It will be a choice between two different paths for America."

The prime-time address came with Obama waging a too-close-to-call race with Romney, who argues that 8.3 per cent unemployment and sluggish growth prove the president is out of ideas and should be sent home to Chicago after one term.

Romney, who made his own convention pitch to voters a week ago, earlier called on Obama to issue a report card to Americans on "forgotten promises and forgotten people."

"Over the last four years, the president has said that he was going to create jobs for the American people and that hasn't happened.

"He said he would cut the deficit in half and that hasn't happened. He said that incomes would rise and instead incomes have gone down.

"This is a time not for him not to start restating new promises, but to report on the promises he made. I think he wants a promises reset."