Here is a look at Republicans who are vying for the party nomination and the opportunity to face Democratic President Barack Obama in the November 2012 election.
Former Minnesota Governor Tim Pawlenty became the first major casualty in the US presidential campaign on Sunday while fellow Republicans Michelle Bachmann and Rick Perry built momentum in the race.
Here is a look at Republicans who are vying for the party nomination and the opportunity to face Democratic President Barack Obama in the November 2012 election.
Mitt Romney
Romney, who lost the nomination to John McCain in 2008, has been viewed as the early front-runner, topping recent polls of potential Republican candidates by an average of about 5 percentage points, according to the website Real Clear Politics.
He also leads the Republican money race, raising $18.25 million in the second quarter for 2011, more than four times as much as any other contender.
Romney co-founded private equity firm Bain Capital and has pushed his business experience as a way to attack Obama's handling of the struggling US economy. Critics say he was a corporate raider who cut jobs.
He is also known for righting the scandal-plagued 2002 Salt Lake City Olympics and a fortune estimated at $250 million.
While favoured by more traditional pro-business Republicans, Romney is viewed sceptically by some conservatives because he was governor of liberal Massachusetts and is a Mormon, a religion some evangelicals do not consider Christian.
Fellow Republicans have attacked him because of a health-care plan he helped develop in Massachusetts that became a model for the Obama healthcare law. Romney has defended the state law while attacking the federal version and promising to repeal Obama's plan.
Michele Bachmann
A leader of the Tea Party movement, Bachmann joined the upper tier of candidates after a strong performance in the first major Republican debate on June 13 in New Hampshire.
A former tax lawyer, Bachmann became the first Republican woman elected to the US House of Representatives from Minnesota in 2006.
A fiscal, social and religious conservative, Bachmann could benefit if former Alaska Governor Sarah Palin decided not to run, because the two are similar in politics and appeal to many of the same voters.
Bachmann has led recent polls in Iowa, where social conservatives are strong, but might struggle in primaries in New Hampshire and Florida, where her strong religious views and uncompromising positions on financial issues may not appeal to more moderate Republicans.
Rick Perry
The three-term Texas governor was second in most polls even before announcing that he would enter the race. He is seen as a candidate who could bridge divides within the party because he is known for promoting job growth and staunch religious conservatism, and is popular within the Tea Party movement.
Perry would have the advantage of being the field's only governor from the South, a powerful party stronghold. Known for his fund-raising acumen, he would also have strong support from within Texas, home to many wealthy Republican donors.
One challenge is the prospect of comparison with another Republican Texas governor known for wearing cowboy boots: former President George W Bush. Bush's lasting unpopularity could pose a hurdle in a general election.
Critics question Perry's economic record. They say many Texas jobs he claims credit for creating are low-wage and his record includes heavy education cuts, low public service levels and high numbers of people without health insurance.
His strong evangelical Christian views could lessen his appeal to independent voters, whose support will be essential to beat Obama. Perry held a religious rally on Aug. 6 whose backers included groups criticised as extreme and intolerant.
Jon Huntsman
He annoyed the White House by resigning in April as Obama's ambassador to China to consider whether to seek the Republican presidential nomination.
Like Romney, Huntsman is a Mormon. The former governor of Utah and member of a wealthy chemicals family is a moderate, which may make it hard for him to win over conservatives who play a big role in the nominating process.
Huntsman's name recognition is low and his biggest immediate hurdle among Republican voters is his service to the Obama administration. He lags far behind in opinion polls.
Sarah Palin
Palin, the party's vice-presidential nominee in 2008, has not said whether she will run but told an interviewer that she expects to make a decision in August or September.
She has star power and can afford to enter the race relatively late because of her broad name recognition.
Palin made herself a millionaire with two books, the TV show "Sarah Palin's Alaska" and paid speaking engagements.
A leading voice in the conservative Tea Party movement, Palin enhanced her influence by campaigning for its candidates in the 2010 congressional elections.
Palin is not a favourite of establishment Republicans who fear her low approval ratings with the broader electorate could doom the party in a general election matchup with Obama.
Ron Paul
An anti-war Republican congressman from Texas who ran unsuccessfully for the party's 2008 nomination, libertarian Paul, 75, is known as the "intellectual godfather of the Tea Party."
His calls for steep cuts in the federal deficit and the size of government have moved to the mainstream of debate in Congress since November when the fiscal conservative movement swept Republicans back into power in the House.
Newt Gingrich
Leading members of Gingrich's campaign team resigned in June, and he has had a poor showing in opinion polls.
The former speaker of the House was the main architect of the 1994 Republican congressional election victory and author of the "Contract with America" political manifesto. Gingrich ended his 20-year congressional career after Republican losses in 1998 elections.
He has faced concerns among religious voters about his personal life. Gingrich is married to his third wife, with whom he had an affair while married to his second.
Rick Santorum
Santorum, once a leading Senate Republican, was badly defeated in his 2006 re-election bid.
He made a name for himself opposing abortion rights and gay marriage while backing welfare reform. He has campaigned hard to enhance his profile in early voting states but remains far back in the Republican field.
Heman Cain
A radio talk show host and former chief executive officer of Godfather's Pizza, Cain was chairman of the Federal Reserve Bank of Kansas City's board of directors and has never been elected to political office.
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