More than a dozen potential Republican candidates are contemplating White House bids in 2016 in what's shaping up as a crowded and diverse field.

A look at some of the challenges facing them:

TED CRUZ

His uncompromising positions have made him a hero among tea party faithful, but the first-term Texas senator must find a way to stand out in a field that includes several like-minded conservatives, as well as appeal to the independents and moderate voters who largely decide general elections.

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MARCO RUBIO

The 43-year-old first-term senator will need to start any campaign by escaping the considerably large shadow of his political mentor, Jeb Bush. While having failed to impress at times when given high-profile opportunities, Rubio's forceful opposition to President Barack Obama's moves to restore relations with Cuba is a sign of his potential to make a mark.

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JEB BUSH

The early favorite of the Republican establishment, the former Florida governor is sure to face resistance from voters about sending a third member of the Bush family to the White House. Even his mother, former first lady Barbara Bush, while recently more supportive, said in 2013 that "we've had enough Bushes."

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CHRIS CHRISTIE

The New Jersey governor appears to have survived a political scandal involving a scheme to create traffic jams on a bridge that links New Jersey with Manhattan. At one point, it had threatened to destroy his presidential ambitions. Still, it reinforced the perception that Christie's tactics sometimes cross the line into bullying. He will have to overcome that perception in a campaign that will draw considerable attention to his tough-talking personality.

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RAND PAUL

The first-term Kentucky senator's greatest strength may also be his greatest liability: his father, former Texas congressman and two-time presidential candidate Ron Paul. Rand Paul is expected to inherit his father's deeply loyal base of supporters. But the senator also will have to distance himself from his father's unorthodox positions, particularly on foreign policy, to have a real shot at winning the GOP's presidential nomination.

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BEN CARSON

The retired neurosurgeon has caught fire with conservatives and has a base of vocal supporters who are starting to organize on his behalf. But Carson, the only African American in the early GOP field, must convince the broader electorate that his career in medicine and willingness to criticize President Barack Obama qualifies him to lead the nation.

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MIKE HUCKABEE

He won the Iowa caucuses in 2008 and continues to enjoy high name recognition because of a consistent media presence, but the former Arkansas governor must expand his support beyond the GOP's evangelical wing to become a real factor in 2016. He just announced that he was leaving his Fox News talk show as he considers whether to seek the nomination.

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RICK SANTORUM

The former Pennsylvania senator whose main focus is social issues likely exceeded his own expectations in the 2012 White House race. Like Huckabee, he always will be popular among Christian conservative voters, but he must appeal to the broader electorate inside and outside his party.

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PAUL RYAN

The 2012 vice presidential nominee's greatest challenge may be convincing people that he seriously is considering running. Ryan's passion is policy, and he is set to play a central role in the tax debate as the new chairman of the House Ways and Means Committee. It will be hard to do that and run for president at the same time.

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BOBBY JINDAL

The Louisiana governor has signaled strong interest in a White House bid, but he needs to find a consistent message while working to convince his party elders — and voters to whom he is largely unknown — to take him seriously.

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RICK PERRY

With a criminal case pending, Perry could struggle to raise money once he leaves the Texas governor's mansion this month. His "oops" moment from the 2012 contest, however, may loom larger over his presidential ambitions and probably will until he proves himself on the debate stage.

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SCOTT WALKER

Wisconsin's governor has built a national profile for taking on his state's public service unions and winning three elections in four years. Still, it is a huge jump to presidential politics for a man who was investigated by state prosecutors searching for campaign finance violations and has yet to be vetted nationally.

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JOHN KASICH

He is a former congressman and now a two-term governor of Ohio, a critical swing state. Kasich's resume is impressive. But his sometimes-unorthodox personality won him few friends in Washington, and it is unclear how his approach would play in a national campaign.

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CARLY FIORINA

Should she run, the former Hewlett-Packard CEO probably will be the only woman in the GOP field. She lost a Senate race in California in 2010 and has a big challenge in proving she can do more than play a symbolic role in the 2016 primary.

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