Republican presidential candidates gather at Ronald Reagan's library in California on Wednesday for the first in a series of debates over the next six weeks that will help define the Republican 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.
The three-term Texas governor, popular with the Tea Party movement, is seen as a candidate who could bridge divides within the party because of his record on promoting job growth and his staunch religious conservatism.
He is the only governor in the 2012 race from the South, a powerful party stronghold, and he is known for his fund-raising acumen in Texas, home to many wealthy Republican donors.
One challenge will be comparisons 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, saying the growth in Texas jobs that he promotes did not result from his policies and that many are low-wage or government jobs. His state record also includes heavy education cuts, low public service levels and high numbers of people without health insurance.
His strong evangelical Christian views could damage his appeal to independent voters, whose support will be essential to beat Obama. Perry held a religious rally on August 6 backed by groups who have been criticized as extreme and intolerant.
Romney, who lost the nomination to John McCain in 2008, had been viewed as the early front-runner until Perry's debut in the race knocked him off his perch.
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 U.S. 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 has a fortune estimated at $250 million.
While favored by more traditional pro-business Republicans, Romney is viewed skeptically 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 healthcare 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.
A leader of the Tea Party movement, Bachmann moved into the race's top tier with a victory last month in the Iowa straw poll, a test of early strength in the state that holds the first nominating contest.
A former tax lawyer, Bachmann became the first Republican woman elected to the U.S. House of Representatives from Minnesota in 2006.
A fiscal, social and religious conservative, Bachmann led early polls in conservative Iowa but has seen Perry take a slight lead there since he entered the race last month.
While battling Perry for conservative support, she also faces a challenge in broadening her appeal and competing in states like New Hampshire and Florida, where her strong religious views and uncompromising positions on financial issues may not appeal to more moderate Republicans.
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.
He resigned in April as Obama's ambassador to China as he prepared his bid for the Republican presidential nomination.
His name recognition is low and he has lagged behind in opinion polls, but he has begun to distinguish himself from the rest of the Republican field by criticizing some of their conservative views.
He drew attention with a Twitter message last month saying he believed in evolution and believed scientific evidence of global warming, adding: "Call me crazy."
Palin, the party's vice presidential nominee in 2008, has not said whether she will run but told reporters during an appearance at the Iowa state fair last month that she will decide by the end of September.
She has star power and can afford to enter the race relatively late because of her broad name recognition, but she has so far made no public effort to prepare for the campaign.
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, she enhanced her influence by campaigning for its candidates in the 2010 congressional elections.
Palin is not a favorite of establishment Republicans who fear her low approval ratings with the broader electorate could doom the party in a general election matchup with Obama.
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.
Leading members of Gingrich's campaign team resigned in June, and he has lagged in the single digits in opinion polls of the race.
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.
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.
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.