Congresswoman Michele Bachmann formally announced her 2012 Republican candidacy for president on Monday after emerging from relative obscurity as one of the most energetic hopefuls in the Republican field.
Here is a look at Republicans who could win the party nomination and face Democratic President Barack Obama in the general election.
Romney, 64, who lost the nomination to John McCain in 2008, leads many polls of potential Republican candidates and is viewed as the party's early front-runner.
He declared his candidacy on June 2 after spending months creating a network of supporters and wealthy donors, particularly in early voting states including New Hampshire. In May, he raised $10.25 million in Las Vegas in a single day.
Romney, who co-founded private equity firm Bain Capital, has pushed his business experience as a way to attack Obama's handling of the U.S. economy. His fortune has been estimated at up to $250 million. Critics say he cut jobs as a corporate raider.
A former Massachusetts governor, Romney is also known for righting the scandal-plagued 2002 Olympics in Salt Lake City.
A vulnerability of his candidacy is the healthcare plan he helped develop in Massachusetts that became a model for the 2010 Obama healthcare law conservatives want to scrap. Romney has defended the state law while attacking the federal version. He has said he would repeal Obama's plan.
A Mormon, Romney might struggle to win support from evangelical Christians.
Huntsman annoyed the White House by resigning in April as Obama's U.S. ambassador to China to consider whether to seek the Republican presidential nomination.
Huntsman, 51, entered the race on June 21 and, like Romney, has roots in Utah and is a Mormon. A former governor of Utah, Huntsman is a moderate, which may make it hard for him to win over conservatives who play a key 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.
Huntsman paints his knowledge of China, America's main global commercial rival and foreign lender, as a strength.
Former Alaska Governor Palin, 47, has not said whether she will run.
The 2008 Republican vice presidential nominee has star power and 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 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.
She remains vulnerable to public gaffes, most recently saying the legendary American patriot Paul Revere "warned the British" not to mess with the colonists. Revere's famous ride was to warn colonial militias that the British were coming.
Palin can afford to enter the race relatively late because of her powerful name recognition.
A leader of the Tea Party movement, Bachmann has joined the upper tier of candidates after a strong performance in the first major debate on June 13 in New Hampshire. She was only 1 point behind first-place Romney in a June 26 Iowa poll.
Bachmann, 55, is a former tax lawyer who is a conservative both fiscally and socially, who is strongly opposed to gay marriage and abortion.
In 2006, she became the first Republican woman elected to the U.S. House of Representatives from Minnesota. Bachmann has been seen as someone who would benefit if Palin decided not to run, as the two are similar in politics and personality.
She is expected to do well at the caucus in Iowa, where she was born and where conservatives are strong. But she might struggle in primaries in New Hampshire and Florida.
The former Minnesota governor joined the national stage in 2008 when his name showed up on John McCain's short list to be the Republican vice presidential candidate.
"T-Paw" -- as he is known by supporters -- was a popular two-term governor in a swing state, giving him credibility as a Republican who can attract vital support from independents.
He won plaudits for eliminating a $4.3 billion state budget deficit without raising taxes. But critics say he used short-term patches to paper over budget holes and blame him for a multi-billion deficit that took shape after he left office.
Pawlenty, who stepped into the race in May, has been a staunch voice against abortion and human embryonic stem-cell research.
As a presidential hopeful, Pawlenty, 50, has tried to raise his national profile with Republican voters by promising "hard truths" about U.S. fiscal woes yet offering generous tax cuts.
Critics say he lacks charisma, a concern undiminished by a debate performances in New Hampshire and South Carolina.
Leading members of Gingrich's campaign team resigned on June 9 in a blow to his 2012 election hopes. His campaign has sputtered from the beginning and the mass exodus could scare off would-be financial contributors and other supporters.
The former speaker of the House, 68, 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 tried to ease concerns among the religious right about his personal life. Gingrich is married to his third wife, with whom he had an affair while married to his second wife.
After announcing his candidacy, Gingrich flew straight into trouble by criticizing a House Republican Medicare reform plan. He apologized after being criticized by fellow conservatives.
Then came revelations that Gingrich and his wife had a large credit line at Tiffany's. Gingrich then disappeared from the campaign trail and went on a Greek cruise with his wife.
Santorum, 53, once a leading Senate Republican, was badly defeated in his 2006 re-election bid.
A favorite of social conservatives, he made a name for himself a decade ago by opposing abortion rights and gay marriage while backing welfare reform. He has campaigned hard to enhance his profile in early voting states.
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 last November when the fiscal conservative movement swept Republicans back into power in the House.
An obstetrician, Paul has also gained new prominence as chairman of a congressional panel that oversees the Federal Reserve, a system he wants to abolish.
A radio talk show host and former CEO of Godfather's Pizza, Cain, 65, was chairman of the Federal Reserve Bank of Kansas City's board of directors and has never been elected to political office.
(Reporting by David Morgan, Ros Krasny, Steve Holland, Alistair Bell, John Whitesides, Patricia Zengerle)