The field of candidates having a White House run in 2016 is a wide one, although it's narrowing by the weeks. Here is a rundown of who is, was and might be.

REPUBLICANS

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Donald Trump

outspoken billionaire business mogul

estimated net worth of nearly $9bn

leads the polls nationally and wins in New Hampshire and South Carolina

a string of controversies include calling on a Muslim ban to US

accused Mexican immigrants of being criminals and rapists

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Ted Cruz

Texas senator and conservative firebrand

gave up Canadian dual citizenship in 2014

shot to fame in 2014 by speaking in Senate for 21 hours against Obama healthcare law

won in Iowa, where he has huge evangelical support

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John Kasich

currently two-term governor of Ohio

former member of the House of Representatives

record of fiscal conservatism and experience with national security

focus on Christian faith for policy-making

Media playback is unsupported on your device Media caption US Senator Marco Rubio: America "is divided on many political issues"

Marco Rubio - DROPPED OUT

Florida senator since 2011

born in Miami to Cuban immigrants

his part in a bipartisan immigration reform package has cost him some right-wing support

conservative on fiscal and social issues

Is this man Hillary Clinton's worst nightmare?

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Ben Carson - DROPPED OUT

author and neurosurgeon

became a popular conservative figure after National Prayer Breakfast speech in 2013

has likened Obamacare to slavery and doesn't believe in evolution

spent a few weeks atop the polls with Trump but since faded

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Jeb Bush - DROPPED OUT

brother to one ex-president, son of another

was Florida governor for eight years

has large war chest with Wall Street backing

criticised for a lacklustre campaign but more feisty in recent weeks

Read full profile

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Carly Fiorina - DROPPED OUT

former boss of Hewlett Packard and executive at AT&T

was on cover of Fortune's Most Powerful Women issue in 1998

says she's running because federal government needs fixing

enjoyed some stand-out debate moments but only has single-digit support

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Chris Christie - DROPPED OUT

lawyer from Newark, New Jersey

was a US federal attorney, now New Jersey governor

moderate Republican with an abrasive edge

unafraid of reaching across aisle in interests of New Jersey

Read full profile

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Scott Walker - DROPPED OUT

governor of Wisconsin

battled unions over bargaining rights and survived recall election

has qualities that appeal to both Republican establishment and Tea Party

scores well in Iowa and is considered by many as dark horse

The union-busting president?

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Lindsey Graham - DROPPED OUT

has extensive foreign policy experience, chairing several Senate subcommittees related to security

spent 30 years in the Air Force Reserve

advocate for Second Amendment rights

calls President Barack Obama's foreign policy tactics "a failure"

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Mike Huckabee - DROPPED OUT

former governor of Arkansas

ordained Southern Baptist minister and social conservative

unsuccessfully ran for nomination in 2008, but won Iowa caucus

raised eyebrows recently when he labelled Beyonce's music as "mental poison"

Does Mike Huckabee want to be president?

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Bobby Jindal - DROPPED OUT

governor of Louisiana

recently said Rand Paul is "unsuited" to be commander in chief

says he is tough on terrorism and "radical Islam"

served in George W. Bush's administration in the Department of Health and Human Services

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George Pataki - DROPPED OUT

former three-term governor of New York

only supporter of abortion rights in Republican field

advocate for gay rights

compared to New York Republican Rudy Giuliani, who did not do well in last presidential race

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Rand Paul - DROPPED OUT

an ophthalmologist and Kentucky senator

a libertarian and Republican, also son of Ron Paul, who ran for president several times

attacked by some in his party as being isolationist on foreign policy

has testy relationship with senior party figures like John McCain and Mitch McConnell

Listen to a BBC Radio profile

The danger and promise of Rand Paul's father

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Rick Perry - DROPPED OUT

Texas governor from 2000 to 2015

supported anti-sodomy laws and gun rights

2012 White House campaign ran aground when he suffered a blank in a TV debate

indicted by a grand jury in 2014 on state corruption charges

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Rick Santorum - DROPPED OUT

Christian conservative from Pennsylvania

second time running for president

strong emphasis on traditional families, anti-gay marriage and anti-abortion

talked about supporting middle-class working families in announcement speech

DEMOCRATS

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Hillary Clinton

former First Lady and Secretary of State

has stumbled amid a scandal involving her use of private email

lost a bruising primary in 2008 to Obama

few experts see her being denied Democratic nomination

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Bernie Sanders

independent senator from Vermont, caucuses with the Democrats

identifies as democratic socialist, has been in Congress for more than 20 years

critic of current campaign finance system, including Citizens United decision

polling well in New Hampshire but dominance of national security as an issue does not suit him

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Martin O'Malley - DROPPED OUT

former governor of Maryland and mayor of Baltimore

has called for stronger regulation of Wall Street

performed well in Democratic debates but failed to make an impression in polls

been very critical of "demagogue" Donald Trump

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Lincoln Chafee - DROPPED OUT

former senator and governor of Rhode Island

was a Republican, then an independent, now Democrat

backs same-sex marriage, higher minimum wage and tax increases on the rich

was the only Republican senator to vote against Iraq invasion

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Jim Webb - DROPPED OUT

former Virginia senator and US Secretary of Navy

author and war novelist

Vietnam veteran

campaigned for Senate wearing the boots his son wore in the Iraq war

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Joe Biden - DECLINED TO RUN

Vice-President of the United States

former long-time senator with foreign policy and infrastructure focus

known for off-the-cuff remarks and sometimes over-familiar public presence

would be 73 on Election Day 2016

Biden rules out White House run