Quick facts on the candidates - those withdrawn included - in the Republican race to face Barack Obama in the November US presidential election:
Delegate count - 415
Top office held - (former) Governor of Massachusetts
Policy position - He has often accused Obama of worsening US economic woes and portrays his biggest asset to be his knowledge of the economy, which comes from a successful business career. But Romney's supposedly clean business record has come under serious scrutiny after rivals accused him of hiding his tax details, which he has since released, having paid an effective 13.9 per cent tax rate.
Many traditional conservatives don't consider Romney to be conservative enough due to his moderate past positions on the key issues of abortion and healthcare.
Delegate count - 105
Top office held - (former) Speaker of the US House of Representatives
Policy position - He has a strong following among cultural conservatives. He is staunchly opposed to abortion and he has called for an immediate halt to public funding for abortion providers.
Gingrich continues to be saddled with baggage involving his private life. He has married three time, and has had two extramarital affairs. Gingrich was forced to resign as speaker in part due to a reprimand from the House Ethics Committee regarding financial irregularities. On foreign policy, he is an advocate of a hardline stance against Iran for its alleged nuclear weapons programme and has called Palestinians an "invented people".
Delegate count - 47
Top office held - US Congressman from Texas
Policy position - He backs small government, non-interventionist foreign policy, a "sound" currency backed by precious metals, expansions of civil liberties and the abolition of the federal income tax.
He has also called for the dismantling of the Federal Reserve, has vociferously spoken against attacking Iran and has called for an end to military aid for Israel.
|Rick Santorum WITHDREW April 11
Delegate count - 176
Top office held - (former) Senator from Pennsylvania
Policy position - He was a big draw among Christian evangelicals and social conservatives owing to his anti-abortion and anti-gay policies. He staunchly opposed same-sex marriage and wanted to penalise employers who hire illegal immigrants - as many of the other candidates would like to do.
In the foreign policy realm, he has called for the expansion of NATO into formerly communist Eastern European countries.
Santorum had been running a distant second to former Massachusetts governor Mitt Romney, but had vowed to stay in the hunt ahead of the primary in his home state later in April.
His decision to quit was partly influenced by a serious illness suffered by his three-year-old daughter, Bella, but possibly also by the huge embarrasment he would have endured if he lost the vote in Pennsylvania.
|Rick Perry WITHDREW January 19
Top office held - Governor of Texas
Policy position - He supported reduction of the business tax burden and regulatory climate and called President Barack Obama a socialist. He even called for troops to guard the US border with Mexico.
Perry led the polls after he jumped into the race in August, but committed a series of gaffes on the campaign trail and in debates. He struggled to defend past statements about Social Security, which he had called a "ponzi scheme". He also failed to recall the three federal department that he had said he would shut down if he were elected.
|Jon Huntsman WITHDREW January 16
Delegate count - 2
Top office held - (former) Governor of Utah
Policy position --The highly-rated and charismatic Republican technocrat quit as ambassador to China and was considered by many as one of the dark horses in the race.
His foreign policy focus was Asia-Pacific, specifically China. He said Beijing was the biggest threat to the US on the world stage. He also said the US should redouble its efforts to broker an Israeli-Palestinian peace deal while honouring Israeli security concerns.
|Michele Bachmann WITHDREW January 4
Top office held - US Representative from Minnesota
Policy position - The current chair of the Congressional Tea Party Caucus, she is avidly pro-life. She supported traditional family values.
On foreign policy issues, she backed a muscular US image around the world.
|Herman Cain WITHDREW December 3
Top office held - former CEO of Godfather's Pizza
Policy position - The Georgia businessman rose to the top ranks of Republican nominating race after he came up with his 9-9-9 economic growth and tax plan.
But he was forced to quit after he was accused of sexual harassment by former employees.