The former Massachusetts governor, who lost out to Senator John McCain for the 2008 Republican nomination, has benefited from strong name recognition to lead early opinion polls this time around, but the race is still wide open.
"I'm Mitt Romney, I believe in America, and I'm running for president of the United States," Romney told a crowd of flag-waving supporters gathered at a farm in New Hampshire, a key early voting state in the nomination process.
SPENDING CAP
Romney ambitiously vowed that as president he would cap federal spending at 20 percent or less of GDP as he attacked Obama for pursuing European-style, big government answers to America's economic problems.
In 2008 voters "gave someone new a chance to lead; someone we hadn't known for very long, who didn't have much of a record but promised to lead us to a better place," he said.
"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."
Democrats hit back by releasing a video featuring footage of Romney giving contradictory opinions on key policy matters. "Romney 2012. Same candidate. Different positions. Again," ran the video tagline.
Contributing to his 2008 nomination failure were doubts from core conservatives that he truly shared their values, as well as questions fueled by his moderate governing style in deeply liberal Massachusetts.
After vowing to cut taxes, be tough on illegal immigration and becoming an outspoken critic of gay marriage, Romney was forced to deny claims he was simply a "flip-flopper" who was ditching previously held liberal positions.
HEALTHY COMPETITION
Romney's opponents rarely miss a chance to point at his main weakness -- a state health care overhaul that looks like a blueprint for Obama's nationwide reforms, which are anathema to core Republican voters.
Romney also faces an uphill battle to overcome deep suspicion of his Mormon faith among the Christian right, many of whom consider the Church of Jesus Christ of Latter Day Saints, as it is officially known, heretical.
A Boston venture capitalist and founder of a management consultancy, Romney first entered politics in 1994 when he unsuccessfully stood against prominent Democrat Ted Kennedy for Massachusetts senator.
In 1999 he was brought in to rescue the 2002 Winter Olympics in Salt Lake City, which had become mired in scandal.
He saved the games from financial ruin, then translated the experience into a successful bid for the Massachusetts governorship in 2002.
A large field of divided Republican White House hopefuls could end up helping Obama, who is vulnerable as the economy sputters its way out of the worst downturn since the Great Depression of the 1930s.
Former Minnesota governor Tim Pawlenty and former House speaker Newt Gingrich are the highest-profile Republican candidates to have declared their candidacy thus far.
TEA PARTY CANDIDATE
Ex-ambassador to China Jon Huntsman, a former governor of Utah, is expected to throw his hat into the ring, as is Minnesota congresswoman Michele Bachmann, a flag-bearer of the conservative and libertarian Tea Party movement.
The biggest hype surrounds the possible candidacy of former Alaska governor Sarah Palin, McCain's surprise running-mate pick in 2008.
An ongoing "One Nation" bus tour along the eastern seaboard is fueling speculation that Palin -- who has a canny grass-roots focus and a genius for milking media attention -- will jump into the Republican field.
Palin dined on Tuesday in New York with another outsized and controversial conservative character, Donald Trump, the bombastic real estate mogul and reality television star who dropped his flirtation with a bid last month.
There is no clear favorite in the confusing Republican race.
Topping one recent CNN poll was former New York mayor Rudy Giuliani, who has not ruled out a 2012 bid and was also in New Hampshire on Thursday for a separate Republican fund-raising event.
Former Arkansas governor Mike Huckabee, who finished second in delegate count to McCain in 2008, disappointed supporters last month by saying no, but reportedly rowed back Wednesday and suggested he could still run.
