Mitt Romney urged voters to help him rebuild the US economy and create millions of new jobs as he accepted the Republican presidential nomination.
He asked them to overcome their disappointment in President Barack Obama and join him in restoring the promise of America.
Mr Romney said he would work to unify a divided country that believed President Obama's lofty campaign promises but had lost hope they would be fulfilled.
"What is needed in our country today is not complicated or profound. It doesn't take a special government commission to tell us what America needs.
“What America needs is jobs. Lots of jobs," the former Massachusetts governor told a cheering crowd on the final night of the Republican National Convention in Tampa.
The speech was somewhat overshadowed by a bizarre performance on stage by Hollywood actor Clint Eastwood, who addressed an imaginary Mr Obama in an empty chair.
A surprise guest who appeared shortly before Mr Romney, Mr Eastwood made a rambling speech that puzzled viewers and interrupted a carefully crafted evening that told the story of Mr Romney's life and why he should lead the country.
Earlier in the evening, the convention heard from a series of Mr Romney's friends and relatives, who painted a picture of a humane, compassionate man - part of a three-day effort to humanise a candidate often accused of being cold and formal.
Mr Romney's speech, which launches a two-month official campaign for the 6 November election, was seen by tens of millions of television viewers and gave some their first extended look at the multimillionaire ex-businessman.
His campaign against President Obama has been dominated by the sluggish economy and lingering high unemployment.
Mr Romney, who says his experience as a business executive is the cure for the ailing economy, promised to create 12m jobs.
He drew a sharp comparison between the promise of Mr Obama's election in 2008 and the results of the past four years.
"Today the time has come for us to put the disappointments of the last four years behind us, to put aside the divisiveness and the recriminations," he said. "Now is the time to restore the promise of America."
He said Americans wanted to believe in Mr Obama but had suffered from his failures of leadership.
"Hope and change had a powerful appeal. But tonight I'd ask a simple question: If you felt that excitement when you voted for Barack Obama, shouldn't you feel that way now that he's President Obama?"
"You know there's something wrong with the kind of job he's done as president when the best feeling you had, was the day you voted for him.”