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WASHINGTON (Reuters) - Republican Mitch Daniels, the Indiana governor, has announced he will not seek his party's U.S. presidential nomination in 2012, adding uncertainty to the race to pick a challenger to President Barack Obama.
Here is a list of other prominent Republicans considering a run for the White House.
Romney is probably the closest the Republican field has come to producing a front-runner. He lost to John McCain in the 2008 Republican race but has been plotting a return.
The former Massachusetts governor has been on a cross-country blitz to meet wealthy, well-networked donors and announced in April that he would set up an exploratory committee to begin raising money for a candidacy.
Romney has pushed his business experience as a way to attack Obama's stewardship of the U.S. economy.
But his Achilles' heel could be the healthcare plan he helped develop for Massachusetts, which bears striking parallels with the Obama healthcare overhaul that conservatives want repealed.
Known for his success in the world of finance, Romney has a personal fortune estimated in 2008 at $190-$250 million. But critics say he caused job losses as a corporate raider.
He has been leading some opinion polls against both rival Republicans and Obama in New Hampshire, where he has a vacation home that gives him a natural advantage in the state's key primary next year.
A Mormon, Romney might struggle to win support from conservative Republicans in the South.
The 2008 Republican vice presidential nominee has major star power in the party. Since resigning as governor of Alaska in 2009, she has made herself a millionaire with two books, a TV show "Sarah Palin's Alaska" and many speaking engagements.
A leading voice in the fiscally conservative, anti-establishment Tea Party movement, she enhanced her influence by campaigning for its candidates across the country in last year's congressional elections.
But some Republican heavyweights are leery of Palin because she is a polarizing figure and her support seems limited to conservatives. She sparked controversy when she referred to herself as a victim of "blood libel" after some suggested that her rhetoric could have contributed to an Arizona shooting rampage in January in which a U.S. congresswoman was wounded.
She has no timetable for deciding on whether to run, but she could enter the race relatively late because of her name recognition.
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 his supporters -- was a popular two-term governor in a big swing state, giving him credibility as a Republican who can attract vital support from independent voters.
Pawlenty won plaudits for eliminating a $4.3 billion state budget deficit without raising taxes, although Democrats say he used short-term patches to paper over budget holes. He has been a staunch voice against abortion and stem-cell research.
Critics say he lacks charisma, a concern that remained undiminished after a low-key performance in a TV debate in South Carolina in early May against Republican rivals.
In "Courage to Stand," a memoir timed to coincide with his campaign, Pawlenty denounced "runaway spending" in Washington and blamed Obama for a mountain of debt.
But he has had to account for his former support for a cap-and-trade anti-pollution plan, which is anathema to conservative voters.
"Everybody's got a couple of clunkers in their record. That's one of mine. It was a mistake. I'm sorry. It was dumb," Pawlenty told a New Hampshire audience.
Pawlenty announced in March that he would set up a presidential exploratory committee. He was the first senior Republican to do so.
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 the 1998 elections.
He remains a leading conservative figure, political pundit and accomplished fund-raiser. Gingrich announced plans earlier this month to run for president in 2012. But he got off to a rough start by angering fellow conservatives when he criticized a Republican plan to overhaul the Medicare health insurance program and embraced a key element of Obama's healthcare law.
A Reuters/Ipsos poll in March said Gingrich was viewed favorably by 35 percent of voters but unfavorably by 38 percent.
He is taking steps to ease concerns among the religious right about his personal life. Gingrich is married to his third wife, Callista, with whom he had an affair in the mid-1990s while he was married to his second wife.
Huntsman annoyed the Obama White House by resigning at the end of April as U.S. ambassador to China to consider a race for the Republican presidential nomination.
Like Romney, Huntsman has roots in Utah and is a Mormon. A former governor of Utah, Huntsman is a moderate Republican, which may make it difficult for him to win over conservatives who play a large role in the nominating process.
But his biggest immediate hurdle is his service to the Obama administration, which the White House has tried to exploit by describing him as a supporter of Obama's policies.
In his first public speech after leaving the ambassador's post, Huntsman addressed the issue by presenting his experience as an example of how citizens should serve their country when asked.
The representative from Minnesota is a leading figure in the Tea Party movement that helped Republicans win control of the U.S. House from Democrats last year.
First elected to Congress in 2006, Bachmann's attacks on the Obama administration during the 2010 congressional elections rose her to prominence in the Tea Party movement.
She is seen as someone who would benefit if Palin decided not to run, as the two appear similar in terms of their politics and personalities.
Bachmann is planning to file papers to form a presidential exploratory committee by June.
Santorum, once a top Senate Republican leader, was defeated in his 2006 re-election bid.
A social conservative, he made a name for himself a decade ago by opposing abortion rights and gay marriage and has campaigned hard to enhance his profile in early voting states.
Santorum formed a presidential exploratory committee earlier in May and recently won a presidential straw poll at a South Carolina Republican Party dinner, largely by being the only prospective candidate to show up for the event.
Paul, an anti-war Republican congressman from Texas who ran unsuccessfully for the party's 2008 nomination, launched his third bid for the White House on May 13, saying the popularity of his fiscal conservative stance shows "the time has come".
Paul, a libertarian, has built a loyal following of activist supporters with his calls for cuts in defense spending and his criticism of the Federal Reserve since the days in 1988 when he first ran for president as Libertarian Party nominee.
Cain, a radio talk show host and former CEO of Godfather's Pizza who announced he would run on Saturday, was chairman of the Federal Reserve Bank of Kansas City's board of directors and has never been elected to political office.
Reporting by Steve Holland, Alistair Bell, David Morgan John Whitesides and Timothy Gardner; editing by Jackie Frank