By Sam Cabral, Matt Murphy & Anthony ZurcherBBC News, Washington
After months of campaigning, the list of Republican candidates for president has been whittled down to just five competitors.
But with one candidate, Donald Trump, completely dominating the field, does anyone else stand a chance?
The eventual winner will challenge the presumptive Democratic nominee, President Joe Biden, in November’s general election.
Let’s look at the main candidates, what they stand for and what they are doing ahead of the Iowa caucuses taking place on 15 January.
Donald Trump is sitting in the driver’s seat for the final stretch before the caucuses – exactly where he has spent most of the past year.
Despite a turbulent four years in the White House, the 77-year-old has an iron-clad grip of much of the party base as well as a commanding lead in national polls. His closest challenger, Florida Governor Ron DeSantis, trails by more than 30 points in recent polls.
If he can translate that lead into success in Iowa, he will be well-positioned to deliver an early knock-out blow to the rest of the field.
Mr Trump has vowed to finish what he started if he is returned to the White House, including undoing the Affordable Care Act and expanding his southern border wall.
But his rhetoric has frequently caused concern on the campaign trail. He has also consistently repeated false claims that he won the 2020 election and accused Mr Biden of leading a “witch hunt” against him.
Florida Governor Ron DeSantis was once viewed as the candidate most capable of defeating Mr Trump, but he has since faded.
Serving two terms as a little-known member of the House of Representatives, the former naval officer was boosted to the governorship by Mr Trump’s endorsement in 2018. There, he has backed a range of conservative legislation that has included restricting abortions and loosening gun laws.
After romping to re-election last year by more than 1.5 million votes, the largest margin in the state in more than 40 years, he was touted as the man to carry forward Mr Trump’s “America First” movement.
But an awkward personal brand, campaign trail flubs, financial woes and an onslaught from the Trump camp have hurt his once-promising bid.
Now Mr DeSantis is staking his entire political fortune on Iowa. He has invested heavily in grassroots organising and nabbed endorsements from Iowa Governor Kim Reynolds and evangelical leader Bob Vander Plaats. Without a strong second-place showing, at least, his campaign may totally collapse.
Nikki Haley was the first major Republican candidate to launch a campaign against Mr Trump, jumping in last February.
Born in South Carolina to Punjabi Sikh immigrants, Ms Haley, 51, became the youngest governor in the country in 2009. She earned national attention by calling for the removal of the Confederate flag from the South Carolina capitol.
Despite saying she was “not a fan” of Mr Trump in 2016, she later accepted his nomination to be US ambassador to the United Nations, a tenure marked by her dramatic exit from a security council meeting while a Palestinian envoy spoke.
At events and debates, the race’s lone woman has sought to find middle ground on hot-button issues and demonstrate her foreign policy expertise. She has also called for a “new generation” of leaders and for mental competency tests for politicians over 75 years old,
Her performance, along with an endorsement from conservative powerhouse Charles Koch, has pushed her poll numbers up since the first debate in August. She is now closing the gap with Mr DeSantis in Iowa.
Her path to winning the nomination relies on being competitive in Iowa and then pulling off a surprise in New Hampshire, where independents can vote in the Republican primary.
Vivek Ramaswamy, 37, launched his dark-horse bid during a late February appearance on the Fox News channel.
An Indian-American biotech entrepreneur with no previous political experience, he was a regular fixture on Fox host Tucker Carlson’s daily programme.
Mr Ramaswamy argues the country is in the midst of an identity crisis driven by declines in faith, patriotism and meritocracy. He recently drew controversy by plunging into a host of conspiracy theories during the final Republican debate.
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He has also positioned himself as a fervent defender of Mr Trump, even vowing that, if elected, he would pardon the ex-president of any crimes.
Mr Ramaswamy was the summer’s pop hit with both the media and conservatives, but his star has since faded. He’ll need better than a fourth-place showing in Iowa to prove he is not a one-hit wonder.
Chris Christie announced his candidacy in June at a town hall event in New Hampshire – which traditionally holds one of the earliest contests in the primary race.
Mr Christie, 60, served two terms as New Jersey governor from 2010-18. Though massively popular in his first term, his second was overshadowed by political scandals involving bridge lane closures and beach closures. He left office with an approval rating of 14%, the lowest in state history.
After his 2016 presidential bid flamed out, Mr Christie allied himself with Mr Trump, leading the incoming president’s transition team and preparing him for debates against Mr Biden in 2020.
He has become a vociferous critic of Mr Trump since the US Capitol riots. But his campaign’s lack of traction suggests Republican voters have little appetite for an acerbic Trump critic.
Beyond taking on Mr Trump, Mr Christie has focused on domestic issues such as the opioid crisis and inflation.
He has largely avoided investing time or money on trying to win over Iowa’s Trump-friendly electorate. Instead, he is betting on New Hampshire, where he will need Ms Haley to stumble.