Republican presidential candidate Mitt Romney will seek to persuade Americans that he was the best hope to lift the struggling US economy and "restore the promise of America".
Mr Romney's speech accepting his party's nomination is the climax of the three-day Republican National Convention and a milestone in his long, often-rocky quest for the presidency.
He will claim the honour he was denied when John McCain won the nomination four years ago.
This year, he had to fend off a series of Republican challengers, questions about his shifting positions and mutterings about his Mormon religion.
The ultimate prize, the White House, will be determined in a November vote.
Polls show Mr Romney and President Barack Obama in a dead heat with the economy the biggest issue in the campaign.
The United States is struggling with 8.3% unemployment and the slowest economic recovery in decades.
Mr Romney was planning to note the excitement Mr Obama's candidacy generated four year ago and say he hasn't delivered on his promise of "hope and change."
"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," he said in excerpts released ahead of the speech.
Prior to his speech, the crowd in Florida applauded wildly as veteran actor and director Clint Eastwood urged America to vote for Mr Romney.
Eastwood urged the US to vote for a president with a business background and not a lawyer, like Mr Obama.
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