Media playback is unsupported on your device Media caption Mitt Romney: "Barack Obama has failed America"

Former Massachusetts governor Mitt Romney has entered the race to be the Republican candidate for US president in 2012, saying President Barack Obama has "failed America".

Mr Romney blamed Mr Obama for the economic woes frustrating voters, like federal spending and unemployment during a speech in New Hampshire.

If elected, he said he would make the US the world's top job creator.

The wealthy businessman is the current frontrunner in the Republican field.

BBC North America editor Mark Mardell, in New Hampshire, says Mr Romney - a tremendous fund raiser and serious seasoned campaigner - is clearly the man for others to beat.

But he is yet to convince conservatives he really is one of them and yet to convince the media that he has the flair and panache need to maintain interest and momentum, our correspondent adds.

'Failed America'

The Republican multi-millionaire was beginning his campaign in the state of New Hampshire, an important early-voting state which is expected to hold its primary election to pick a candidate in February 2012.

Analysis If the delivery was occasionally hesitant, Mitt Romney's message had been painstakingly honed. In this campaign he will style himself as a trustworthy chief executive for the nation - able to draw on vast business experience in righting the perceived economic wrongs of the Obama administration. Team Romney are well aware of their man's past reputation for aloofness, so the pitch was carefully framed in terms of human experience. The multi-millionaire candidate declared himself heart-broken by the struggles of everyday Americans coping with unemployment, repossessions and soaring petrol prices. In shirtsleeves, rather than the 2008 business suit, Mr Romney seemed approachably relaxed. A gloomy critique of Barack Obama was counter-balanced by sunny optimism about America's potential. US voters dislike outright negativity. But as long as the economy continues to splutter, Mitt Romney will make capital from the straight-up contrast between himself and the president. However, his strategy will have to evolve when the inevitable attacks from his own side begin.

Analysts say Mr Romney's speech, which was released ahead of his announcement, is also tailored to appeal to conservatives who hold great sway in choosing the Republican presidential nominee in Iowa and South Carolina.

"Government under President Obama has grown to consume almost 40% of our economy," Mr Romney said.

"At the time, we didn't know what sort of a president he would make.... Now, in the third year of his four-year term, we have more than promises and slogans to go by. Barack Obama has failed America."

Mr Romney has said, if elected, he would balance the federal budget and limit federal spending at 20% of gross domestic product.

Mr Romney's announcement on Thursday marks the second time he will be running for the Republican nomination.

He lost the party's 2008 nomination race to Arizona Senator John McCain and has spent the four intervening years building support within the party and preparing for the 2012 race.

Sarah Palin tour

Our correspondent says that Mr Romney's biggest problem is that as governor of Massachusetts he introduced a type of compulsory health insurance which looks very similar to President Obama's reforms which are heartily loathed by most Republicans.

He also has a reputation for flip flopping - moving from being a relatively liberal Republican to a hard line conservative, our correspondent adds.

He is also a Mormon, which may not bother most Americans, but does concern some evangelical Christians.

The pool of Republican candidates has been slow to solidify, with former speaker of the House of Representatives Newt Gingrich and former governor of Minnesota Tim Pawlenty the only other major Republicans to have announced their candidacy.

During the past week, Texas Governor Rick Perry was thought to be considering a bid, with Representative Michele Bachmann also stepping towards a run.

Meanwhile, former vice-presidential Republican candidate Sarah Palin was also set to appear in New Hampshire on Thursday, though her aides have not said precisely where she will turn up in the state.