As the Republican presidential race enters the critical chapter of state caucuses and primaries, which begin in January 2012, who is running to face President Barack Obama as the Republican opponent in the race for the White House?
Despite regular fluctuations in opinion polls, the Republican field is made up of seven main candidates vying for the presidential nomination.
Here are the leading Republican contenders:
Mitt Romney entered the 2012 Republican presidential race as the presumed frontrunner. And he has maintained that status as one after another challenger has risen to meet him only to self-destruct.
Mr Romney, the former one-term governor of Massachusetts, brought to the race wealth, business experience, a national profile and a broad network of fundraisers and supporters left over from his failed 2008 White House bid.
With his square jaw, gleaming eyes and immobile hair greying at the temples, some see him as a presidential candidate straight from central casting.
Mr Romney lost the party’s 2008 nomination race to Arizona Senator John McCain but took only a brief break from the campaign trail. Almost as soon as Barack Obama was in the White House, Mr Romney began building support for the 2012 contest.
He hopes that his background in business will help him convince voters he can manage America’s halting economic recovery better than President Barack Obama – or other Republicans in the field.
Suspicion of Mormons
But in order to win the Republican party nomination, he must convince primary voters of the authenticity and depth of his conservative principles. He must also persuade them to overlook his relatively liberal record as governor of Massachusetts, a solidly Democratic state.
And the religious conservatives who are influential in the nominating process will have to overcome suspicion about his Mormon faith.
Willard Mitt Romney was born in 1947 in Michigan. His father, George Romney, was later that state’s Republican governor and himself ran for the Republican presidential nomination in 1968.
He served two years as a Mormon missionary in France, then went to Brigham Young University and studied law and business at Harvard.
Later, Mr Romney took a senior position in the Mormon church and joined Boston management consulting firm Bain and Company, soon rising to chairman. He also founded Bain Capital, a venture capital firm affiliated with Bain.
In 1994, Mr Romney attempted to unseat veteran Democratic Senator Ted Kennedy. He lost but raised his profile in the state and among Republicans nationally.
Haunted by healthcare
And in 1999, he was tapped to run the 2002 Salt Lake City Winter Olympics. The preparations had been mired in scandal, and organisers were attracted to Mr Romney’s deep connections within the Mormon church, his business acumen and his reputation for honesty.
The 2002 Olympics were widely viewed as a success. Later that year Mr Romney ran for governor of Massachusetts as a pragmatic centrist, allaying fears he would foist right-wing social policies on the liberal state. He won.
As governor, he signed into law a comprehensive healthcare overhaul that required all Massachusetts residents to obtain health insurance and provided subsidies to those who could not afford it or who did not receive it from their employers.
That policy has thrown Mr Romney on the defensive time and again during his bid for the White House.
Critics have accused the former governor of responsibility for Mr Obama’s 2010 healthcare plan, which is detested by Republicans, and conceptually similar to the programme Mr Romney signed into law in 2006.
Even before he officially announced his candidacy this time round, Mr Romney made a speech defending the Massachusetts policy while attacking Mr Obama’s programme.
Mr Romney has also faced questions about his commitment to social conservative principles.
During his tenure as Massachusetts governor, a court ruling made the state the first to allow same-sex marriage.
Mr Romney gave a qualified criticism of that decision, saying marriage laws should only be altered by a vote of the people, and sought to build support for a state constitutional amendment banning same-sex marriage.
The real thing?
Mr Romney opted not to seek re-election in 2006 in order to explore a presidential run.
In the 2008 race, Mr Romney sought to cast himself as a conservative whose success in Massachusetts indicated he could win over Democratic and independent voters.
But Mr Romney was never able to overcome doubts about his authenticity and accusations he had shifted his positions on abortion and gay rights merely to appeal to the more conservative national Republican electorate.
He pulled out of the race in February after spending $35m (£21.4m) of his own money.
This time around, Mr Romney has sought to remain above the fray by training his fire on the president rather than on his rivals for the Republican nomination.
And his experience from the 2008 race has proven valuable, in particular in a gruelling series of debates this summer and autumn.
His has consistently led the pack in fundraising and in the polls, though his overall level of support has remained tepid.
Doubting Mr Romney’s ideological purity, Republican voters have been inviting other candidates to challenge him from the right.
He may not fire up the party base, but his campaign believes he is the best equipped to defeat Mr Obama in next November’s elections.
And as primary season looms, Mr Romney can only hope Republicans will shrug their shoulders or hold their noses and pick him.
The Texas congressman has won a devoted following among libertarian-minded Republicans with his calls for a return to the gold standard, the abolition of the Federal Reserve and the Internal Revenue Service, and his staunch opposition – unusual in the Republican Party – to the war in Iraq and to American militarism in general.
Supporters of Mr Paul, an obstetrician, gained a reputation during the 2008 race for their enthusiasm for the candidate – as well as for their practice of disrupting rival candidates’ rallies and press conferences.
Some of his backers also became known for espousing far-out conspiracy theories, such as the suggestion the US government was behind the 11 September 2001 terror attacks, tainting his movement in the eyes of the mainstream Republican electorate.
Mr Paul, 75, announced his candidacy in May in remarks that mixed anti-war cries with arguments for the legalisation of heroin and the end of federal flood and disaster insurance programmes.
He will bring to the race a legion of devoted followers who can organise and raise funds.
But to his detractors, Mr Paul is too eccentric and his ideas too fringe for them to take him seriously as a presidential hopeful.
It remains to be seen whether his rising profile in swing states such as Iowa will be reflected at the polls – a development that could upset the dynamic of the race as it moves into the primaries.
In July 2011, Mr Paul announced he would not stand again for his House seat, saying he wanted to remain focused on his presidential bid.
The former Pennsylvania Senator hopes to capitalise on his solid social conservative credentials. He last appeared on the ballot in 2006, when he lost his re-election bid by 17 points.
Polls have shown him in a distant seventh place in the race but just days before the first round of voting in Iowa – where Mr Santorum has campaigned relentlessly – opinion polls show some momentum gathering behind the candidate.
He has attacked rivals such as Ron Paul with gusto but he has a lot of ground to make up on the front-runners.
Mr Gingrich, the former speaker of the House of Representatives, announced he would be running against Mr Obama in 2012 on YouTube, Twitter, and in an interview on Fox News.
Since he left office 12 years ago, Mr Gingrich has built a broad network of conservative businesses and non-profit organisations, generating films, books and position papers, and has sought to align himself as an elder statesman and a creative source of conservative policy ideas.
He remains widely respected in the party for leading congressional Republicans out of 40 years in opposition in 1994, although he lost the speaker’s gavel in 1998 after the party took significant losses.
In June 2011 his chances took a serious knock when senior members of his campaign team walked out, citing differences over strategy.
But Mr Gingrich made a roaring comeback in November, with surging poll ratings in Iowa, South Carolina and Florida – all key early voting states.
But Mr Gingrich has been criticised by fellow party members for having taken positions on several policy issues that they viewed as unpredictable or inconsistent with conservative principles.
For example, he attacked a plan popular among Republicans to slash and privatise a healthcare programme for the elderly.
Mr Gingrich can be both inflammatory and irascible – qualities Americans do not tend to see as presidential – and has a long record of undiplomatic quotes.
His three marriages may also haunt him in a 2012 campaign. His first wife has accused him of divorcing her while she was in hospital recovering from cancer.
Mr Gingrich was having an affair with a staffer (whom he later married) while he was leading the charge to impeach former President Bill Clinton over his affair with Monica Lewinsky.
Rick Perry may have entered the race later than his rivals but he has spent months raising money in Texas, where he has been governor for 10 years.
He was the last of the candidates to formally launch a bid for the White House, making his announcement in the key primary state of South Carolina in August.
Mr Perry’s mantra is small government and he can boast that he balanced the books in the second largest state in the US, although critics complain at the scale of resulting spending cuts, especially in education.
A deeply religious man, Mr Perry sealed his popularity among Christian evangelicals when he led a prayer rally attended by 30,000 people in Houston, Texas, in early August.
So, he appeals to two strong powerbases of the Republican party – the fiscal hawks and the social conservatives.
In the first weeks of his campaign, Mr Perry challenged Mitt Romney’s position in opinion as front-runner, draining support from the other Tea Party favourite Michele Bachmann, who occupies similar political ground.
His ability to fire up the Republican base – a la Bachmann – energised the race, although Mr Perry has struggled to maintain that momentum after a series of weak performances in nationally televised debates.
The Minnesota congresswoman, a favourite of the Tea Party, used the first televised debate in New Hampshire to formally announce that she was entering the contest.
She is an outspoken conservative who has been spending time in the early primary states.
The fiery Ms Bachmann has a small core of staunch support, although wider momentum behind her has slowed since she won the influential Iowa Straw Poll in August.
The Iowa-born 55-year-old has a law degree and worked as a tax attorney, and she has fostered 23 children.
The former governor of Utah joined the race for the Republican presidential nomination with a distinct, if awkward, qualification: he worked for the other side.
As ambassador to China for two years under President Barack Obama, he has arguably the most foreign policy expertise of the Republican field.
But it remains to be seen whether or not that experience – which he defends as loyal service to the country rather than the president’s policies – will be an advantage to Mr Huntsman in his bid to succeed the man who appointed him.
Jon Huntsman, 51, is the motorcycle-driving son of billionaire Jon Huntsman Sr, who founded a large chemical manufacturer.
He dropped out of high school to play keyboard in a rock band, later finishing school and graduating from the University of Pennsylvania. He also served as a Mormon missionary in Taiwan, and is said to speak fluent Mandarin.
His tone has been markedly more moderate than that of his rivals, and he has in the past backed civil unions for same-sex couples and said he believes in the science of climate change.
Since he spent the last two years outside the country, he must now introduce himself to Republican voters who have been steeped in the angry, stridently anti-government Tea Party movement.
Some analysts have suggested Mr Huntsman aims to be the adult in the race, rejecting his rivals’ crowd-pleasing attacks on Mr Obama while counselling the US must make hard choices to rein in the national debt.
Mr Huntsman’s ratings have languished in single-digits in opinion polls and he has not campaigned in the state of Iowa, preferring instead to focus on another key battleground, New Hampshire, also the heartland of Mitt Romney’s support base.
It remains to be seen whether, after four years of Mr Obama in the White House, Mr Huntsman will satisfy voters.
Dream Dare Win