WASHINGTON (Reuters) - Republican voters looking for their party's 2012 challenger to President Barack Obama will choose from a field that has as many perceived weaknesses as strengths.

"It's tough to beat an incumbent, and if you're going to do it, you're going to have to do it with someone who is stellar. It's not clear that Republicans have that," said Julian Zelizer, a political scientist professor at Princeton University.

The candidates who can best manage their negatives are likely to be the ones who will rise from the pack in the months of campaigning, which is just beginning to heat up.

There is no better example than former Massachusetts Governor Mitt Romney, who many consider the front-runner of a race that is wide open and off to a slow start.

His potential Achilles' heel is the healthcare plan he helped develop for his state that looks suspiciously like the Obama plan that conservatives want to repeal.

"Our experiment wasn't perfect -- some things worked, some didn't, and some things I'd change," Romney said in confronting the healthcare problem head-on in a New Hampshire speech last Saturday.

Former Minnesota Governor Tim Pawlenty is expected to run but is so low-key that some analysts wonder if he is too lackluster to be the Republican nominee.

"He's got to make sure he improves the charisma piece of it," said Republican strategist Danny Vargas. "Pawlenty has to beef up the jazz."

Indiana Governor Mitch Daniels, who is considering a run, has spoken eloquently about America's dire fiscal situation but has alarmed some conservatives by calling for a truce on social issues like gay marriage.

Conservative activist Ralph Reed blasted the idea of a truce when speaking Monday to the Iowa Faith & Freedom Coalition, which heard from several potential candidates.

"I'd like to have a leader that can walk and chew gum at the same time," he said.

Mississippi Governor Haley Barbour, pondering a run, has a lobbyist past which, when scrutinized, may be fodder for criticism. Lauded for how he guided his state through the ravages of Hurricane Katrina, he has been accused of lacking sensitivity to the civil rights struggles in his native South.

Former House of Representatives Speaker Newt Gingrich, testing the waters for a presidential campaign, is an ideas man but his two divorces make some conservatives uneasy.

U.S. Ambassador to China Jon Huntsman may be too moderate for conservatives who tend to vote in Republican primary elections. Both Huntsman and Romney are Mormons, which may not sit well with the religious right.

And former Alaska Governor Sarah Palin, who generates enthusiasm among conservatives, is a deeply polarizing figure who has not been able to broaden her base of support. She is considering a run.


Linda Fowler, a professor of government at Dartmouth College in New Hampshire, said candidates can overcome their perceived weaknesses by making voters comfortable with them.

"The big thing with a lot of this is trust and whether you can deal with your supposed flaws in a way that inspires people's confidence so they know what they're going to get," she said.

Americans need only look back to recent presidential elections for evidence.

Democrat Bill Clinton was seen as having had too many marital problems to be elected, but he defeated incumbent President George H.W. Bush in 1992. Obama overcame long odds to win in 2008, despite many saying an African-American who was little known at the start of the campaign would not be elected.

Just like the elder Bush in 1992, Obama at this stage looks to be in good position to win a second term when compared to his field of potential challengers.

Norman Ornstein, political analyst at the American Enterprise Institute, said a party can seem to have a weak field when there is no obvious front-runner.

"That doesn't mean that they can't get there -- especially if there are problems in the economy," he said.

(Additional reporting by Kay Henderson in Des Moines; editing by Philip Barbara)