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.
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.
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.