He identified America's faltering economy as the key to beating Barack Obama, accusing him of having failed to deliver on 2008 campaign promises to turn the economy round. "Now, in the third year of his four-year term, we have more than slogans and promises to judge him by. Barack Obama has failed America."
Romney, who is ahead in polls of Republican voters, is hoping to consolidate his lead by disclosing next month that he outperformed his Republican rivals in fund-raising, having taken in tens of millions of dollars over the last three months.
He has been campaigning informally for at least 12 months before making the official announcement in Stratham, New Hampshire, at Bittersweet Farm, a location that might turn out to be prophetic of his campaign.
He came off second best to John McCain in the Republican race in 2008, in spite of spending $40 million of his own money, and faces formidable problems in 2012. He demonstrated again yesterday he is a poor speaker, with a fondness for cliches, and is viewed with suspicion by right-wingers on health, abortion, gay rights and gun laws. Christian evangelicals may be reluctant to back a Mormon.
He could be vulnerable to a run by a populist candidate such as Sarah Palin, McCain's running mate in 2008. Even though she says has not decided yet whether to join the Republican contest, she has dominated the media all week with a bus tour of the east coast.
Palin, supposedly on a tour to promote US interest in the constitution, mischievously took her bus to New Hampshire yesterday, saying it was only coincidental that she would be there on the same day as Romney's announcement. "Maybe we'll run into him," Palin said.
While Obama is favourite to win in 2012, he is vulnerable to a strong Republican challenge, with the US economy slow to come out of recession. Unemployment is hovering around 9% and no US president since the 1930s has won a second term with figures that high.
In his speech Romney admitted that the economy was in recession when Obama took office, but said he had made it worse and made it last longer. "Three years later, over 16 million Americans are out of work or have just quit looking. Millions more are underemployed. Three years later, unemployment is still above 8%, a figure he said his stimulus would keep from happening," Romney said.
The country was in crisis and he, as a successful businessman, was capable to the "bold" actions needed to resolve it, he said.
He also attempted to woo Tea Party activists by promising to repeal Obama's health reforms."I will insist that Washington learns to respect the constitution … we will return responsibility and authority to the states for dozens of government programmes – and that begins with a complete repeal of Obamacare," Romney said.