Vivek Ramaswamy suspended his campaign to be US president on Monday, cutting the field of Republican candidates to four.
Mr Ramaswamy made his announcement after former president Donald Trump easily won the Iowa caucuses.
The wealthy biotech entrepreneur and conservative author finished fourth, behind Florida Governor Ron DeSantis and former United Nations ambassador Nikki Haley.
Mr Ramaswamy told supporters he is suspending his bid and endorsing Trump.
Here is a look at the candidates still competing for the Republican and Democratic nominations, as well as the third-party contenders:
REPUBLICAN PRIMARY FIELD
– Donald Trump
The former president announced his third campaign for the White House on November 15 2022, at his Mar-a-Lago resort, forcing the party to again decide whether to embrace a candidate whose refusal to accept defeat in 2020 sparked the US Capitol attack and still dominates his speeches.
The Republican front-runner remains hugely popular in the party, despite making history as the first president to be impeached twice and inciting the Capitol insurrection on January 6 2021.
Referring to himself as America’s “most pro-life president”, Mr Trump’s three nominations of conservative judges to the Supreme Court paved the way for the reversal of Roe v Wade, which had legalised abortion nationwide for nearly 50 years.
Sweeping criminal justice reforms he signed into law in 2019 eased mandatory minimum sentences and gave judges more discretion in sentencing.
In March, Mr Trump became the first former US president to be criminally charged, facing 34 felony counts of falsifying business records as part of a hush-money scheme. Since then, he has been charged with 57 more felonies in three other criminal cases, accused of mishandling and unlawfully retaining classified documents and trying to illegally overturn the results of the 2020 election.
– Ron DeSantis
The Florida governor officially launched his 2024 presidential campaign on May 24 in a glitch-marred Twitter announcement.
Heralding his state as a place “where woke goes to die”, Mr DeSantis has framed his campaign around a desire to bring the conservative policies he championed in Florida to the national stage.
He has made a name for himself battling with Disney over the entertainment giant’s opposition to a Bill dubbed by critics the “Don’t Say Gay” law, which bans instruction or classroom discussion of LGBTQ issues in Florida public schools for all age groups.
Under his governorship, the state has also banned abortions after six weeks of pregnancy and blocked public colleges from using federal or state funding on diversity programmes.
– Nikki Haley
The former United Nations ambassador and South Carolina governor became the first major Republican challenger to Donald Trump when she kicked off her campaign on February 15 in Charleston. She is the only woman in the Republican field.
The former Trump cabinet official once said she would not challenge her former boss for the White House in 2024. But she changed her mind, citing the country’s economic troubles and the need for “generational change”, a nod to 77-year-old Mr Trump’s age.
– Asa Hutchinson
The former two-term Arkansas governor launched his presidential campaign on April 26 in Bentonville, pledging to “bring out the best of America” and to reform federal law enforcement agencies.
He announced his campaign shortly after Donald Trump was indicted by a grand jury in New York and has called for the former president to drop out of the race, saying: “The office is more important than any individual person.”
DEMOCRATIC PRIMARY FIELD
– Joe Biden
President Joe Biden formally announced his re-election campaign on April 25 in a video, asking voters for time to “finish this job”.
Mr Biden, the oldest president in American history, would be 86 at the end of a second term, and his age has prompted some of his critics to question whether he can serve effectively.
A notable swathe of Democratic voters have indicated they would prefer him not to run, though he is expected to easily win the Democratic nomination.
Mr Biden, who has vowed to “restore the soul of America”, plans to run on his record.
He spent his first two years as president combating the coronavirus pandemic and pushing through major Bills such as the bipartisan infrastructure package and legislation to promote high-tech manufacturing and climate measures.
– Marianne Williamson
Self-help author Marianne Williamson entered the Democratic primary on March 4 in Washington, calling for “a vision of justice and love that is so powerful that it will override the forces of hatred and injustice and fear”.
During her unsuccessful 2020 presidential campaign, she proposed the creation of a Department of Peace and argued that the federal government should pay large financial reparations to black Americans as atonement for centuries of slavery and discrimination.
– Dean Phillips
The Minnesota congressman is the first elected Democrat to challenge Joe Biden for the nomination.
After months of calling for a primary challenger, Mr Phillips entered the race himself on October 27 with a speech outside New Hampshire’s statehouse.
While he has been effusive in his praise for Mr Biden, the 54-year-old also says Democrats need younger voices to avoid a nightmare scenario where Donald Trump wins another election next autumn.
Mr Phillips is one of the wealthiest members of Congress and heir to his stepfather’s Phillips Distilling Company empire, which holds major vodka and schnapps brands. He once served as the company’s president but also ran gelato maker Talenti.
His grandmother was the late Pauline Phillips, better known as the advice columnist “Dear Abby”.
INDEPENDENT BIDS
– Robert F Kennedy Jr
The best-selling author and environmental lawyer announced on October 9 that he was ending his Democratic presidential bid and instead launching an independent run.
A nephew of President John F Kennedy and son of Attorney General Robert F Kennedy, he initially launched a long-shot bid to challenge Joe Biden for the Democratic nomination on April 19 in Boston. He said in announcing his party switch that he intended to be a spoiler candidate for both Mr Biden and Donald Trump.
Mr Kennedy has emerged as one of the leading voices of the anti-vaccine movement, with public health experts and even members of his own family describing his work as misleading and dangerous. He has also been linked to far-right figures in recent years.
– Jill Stein
The environmental activist, whose 2016 third-party presidential bid was blamed by Democrats for helping Donald Trump win the White House, says she is making another run for the nation’s highest office.
Ms Stein announced on November 9 that she will again run under the Green Party banner.
“I’m running for president to offer that choice for the people outside of the failed two-party system,” she said.
She ran against Donald Trump and Democrat Hillary Clinton in 2016 as a Green Party candidate and received about 1% of the vote.
Some Democrats said her candidacy siphoned votes away from Ms Clinton, particularly in swing states like Wisconsin.
– Cornel West
The progressive activist and scholar announced on October 5 that he was ending his bid for the presidency under the Green Party banner and was instead running as an independent.
He wrote on X, formerly Twitter, that he was running as an independent to “end the iron grip of the ruling class and ensure true democracy!”
He added: “We need to break the grip of the duopoly and give power to the people.”
He initially announced in June that he would be running as a member of The People’s Party before soon switching to the Green Party.
WHO HAS DROPPED OUT?
Republicans:
Former vice president Mike Pence
Senator Tim Scott of South Carolina
Former New Jersey governor Chris Christie
Entrepreneur Vivek Ramaswamy
North Dakota Governor Doug Burgum
Radio show host Larry Elder
Businessman Perry Johnson
Former Texas congressman Will Hurd
Miami Mayor Francis Suarez