The field of Republican candidates seeking their party's presidential nomination has narrowed, but there are still 11 running serious campaigns.
State-by-state intra-party voting begins Monday in Iowa. In July 2016, the conservative party will formally choose a nominee to face against the Democratic candidate in the November 2016 general election.
JEB BUSH, 62, the former governor of Florida, son of former president George HW Bush (1989-93) and brother of former president George W Bush (2001-09.) He is running on his eight-year record as governor, pointing to tax cuts, an improved school system, reduction in violence against women and job growth during his two terms.
BEN CARSON, 64, an African-American neurosurgeon, is a pioneer in separating conjoined twins. Raised in poverty by a single mother, he calls for reform of the welfare state. He came to prominence in 2013 after criticizing Obama's health care reforms.
CHRIS CHRISTIE, 53, New Jersey governor, is a political moderate in a largely Democratic state, earning praise for budgetary discipline. However, critics say his personality is too coarse for the White House.
TED CRUZ, 45, US senator from Texas and leader of the Republican Party's tea party faction, helped orchestrate the 2013 government shutdown. Son of a Cuban exile, he champions stricter immigration laws, small government, protection of individual liberties and lower taxes. Cruz advocates strong US ties with Israel and seeks to appeal to people who oppose abortion, gay marriage and limits on gun ownership.
CARLY FIORINA, 61, was named the most powerful woman in American business by Fortune magazine while chief of Hewlett-Packard. Her only experience in politics is a failed run for the US Senate in 2010 and as an advisor to Senator John McCain's failed 2008 presidential bid.
MIKE HUCKABEE, 60, made some headway in the 2008 Republican race for the presidential nomination but lost the prize to McCain. The former Arkansas governor and ordained Baptist minister is a favourite of the religious right and stays in the limelight as a talk show host on the conservative Fox News network.
JOHN KASICH, 63, has served as governor of Ohio since 2011 and counts his popularity in the state - considered a must-win in presidential elections - as among his biggest assets. Kasich served 18 years in the US House of Representatives, including a stint as chairman of the powerful Budget Committee and points to his success in balancing the federal budget as well as in closing a budget shortfall in Ohio.
RAND PAUL, 53, spent 17 years as an eye doctor and now focuses on the large size of government. The Kentucky senator is an outspoken advocate of free markets and individual liberties and clashes with Republican hawks on defence and foreign policy.
MARCO RUBIO, 44, a first-term senator from Florida is the son of parents who fled Cuba in 1956, and has called for immigration reform while outlining a conservative domestic policy centred on family. Rubio has pledged a robust programme of foreign involvement, advocates tougher stances on China and Russia and opposes current conciliatory moves toward Cuba.
RICK SANTORUM, 57, a former Pennsylvania senator and conservative Catholic, came in second in the Republican primaries in 2012, losing to Mitt Romney. He opposes abortion and homosexuality, and has highlighted his working-class roots in Pennsylvania.
DONALD TRUMP, 69, for years led the campaign of untruths that President Barack Obama is not a US citizen. The billionaire real estate mogul from New York provoked a firestorm of protest over his charges that illegal immigrants are rapists and thieves and, even more recently, for comments questioning McCain's status as a war hero because, he says, McCain did nothing to earn the designation other than being captured.