Barack Obama has failed to deliver on his promise of hope and change and it is time for new leadership in the White House, Mitt Romney told America today, in the most critical speech in his presidential campaign.

Mr Romney accepted the Republican presidential nomination, casting himself as the best hope to lift the struggling US economy and "restore the promise of America".

His speech marked the climax of the three-day Republican National Convention in Tampa, Florida, and a milestone in his long, often-rocky quest for the presidency.

He has had to fend off a series of party 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 Mr 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 said excitement over Mr Obama's promises from his campaign four years ago "gave way to disappointment and division".

"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.

The speech was seen as a national introduction of sorts for the Mr Romney, 65 - an oddity considering his years in the public eye. Yet for all his time as candidate, Massachusetts governor and head of the 2002 Winter Olympics in Salt Lake City, he remains something of an enigma. He is often seen - unfairly, friends say - as stiff and distant.

While polls show voters view the multi-millionaire former businessman as more capable of fixing the economy, they find Mr Obama to be more honest and likeable.

The campaign hopes today's speech and the convention in general will change those perceptions. A portion of the convention stage was rebuilt overnight so Mr Romney would appear surrounded by delegates rather than speaking from a distance, in an attempt to soften his image.

Before his speech, church members warmly presented Mr Romney as a compassionate man who lives his Mormon faith of service.

Grant Bennett described Mr Romney's volunteer work shovelling snow and raking leaves for the elderly and Ted and Pat Oparowski recalled how he befriended their 14-year-old son David as he was dying of cancer.

Republicans also turned to Hollywood firepower, with Clint Eastwood taking a turn at the podium. "When somebody does not do the job you've got to let 'em go," he told a roaring audience.

Mr Romney made a press-the-flesh entrance into the hall, walking slowly down one of the convention hall aisles and shaking hands with dozens of delegates. The hall erupted in cheers when he reached the stage and waved to his cheering, chanting supporters before beginning to speak.

His speech was the traditional convention finale, and thousands of red, white and blue balloons nestled in netting high above the floor, ready to be released on cue once he completed his remarks.

But more than the hoopla, the evening marked one of a very few opportunities any presidential challenger is granted to appeal to millions of voters in a single night.

The party has rallied behind Mr Romney despite long-standing concerns about his shifting political positions and doubts about whether he was a true conservative. His religion also unsettled some evangelicals, a core Republican constituency, who do not see Mormonism as a true Christian faith.

But the Republican desire to evict Mr Obama from the White House overwhelms any trepidation about Mr Romney. Moreover, the party was thrilled when Mr Romney picked congressman Paul Ryan, the architect of a plan to slash government spending, as his vice presidential running mate.

The two-month campaign to come includes other big moments - principally a series of one-on-one debates with Mr Obama.

Democrats looked to use Mr Ryan's speech to fund-raising advantage, highlighting factual errors. In a letter sent to potential donors, Obama campaign manager Jim Messina said the Ryan speech "represents a huge bet by the Romney campaign - they've decided that facts, truth and reality will not be a brake on their campaign message".

The president himself was staying out of the spotlight yesterday. But in an interview with Time magazine he said he hoped for a more productive second term if re-elected, because "the American people will have made a decision. And, hopefully, that will impact how Republicans think about these problems".