The first primary election — in New Hampshire — is more than a year away but already eight Democrats have said they are up for the challenge.
Although five men — John Kasich, Bob Corker, Jeff Flake, Larry Hogan and Bill Kristol — have said they plan to stand for the Republican nomination but Donald Trump is almost certain to win it and run again on 3 November, 2020.
So who will be seeking the Democratic nomination and what are their chances?
THE DEFINITE RUNNERS
- Kamala Harris
The latest entrant into the contest, Harris, 54, has already become the second favourite behind Trump but the task facing her is evident in the odds — she is 8-1 while Trump is as short as 7-4 with some bookmakers.
So who is she?
She is the offspring of two immigrants. Her father, Donald Harris, was born in Jamaica and came to the US in 1961 while her mother Shyamala Gopalan was the daughter of an Indian diplomat.
Her parents divorced when she was a child and her mother took her and her sister Maya — now a successful attorney in her own right — to Montreal, where she taught at McGill University.
After studying political science and economics at university Kamala returned to California, trained to become a lawyer and in 1990 became a deputy district attorney.
In 1994 she met and dated Willie Brown, a prominent San Francisco Democrat politician, for a while. She was 29. He was 60.
Kamala, who later married lawyer Doug Emhoff, served two terms as district attorney of San Francisco before becoming the first ethnic minority woman attorney general of California.
Harris was elected as a senator in 2017 and has impressed in Congress but she has also faced criticism for saying she was not aware of allegations of sexual harassment against one of her aides, who resigned in 2016.
Harris is an intelligent, attractive woman who could does not have the baggage of a Hillary Clinton and could certainly hold her own against Trump in a televised presidential debate.
"I have the unique experience of having been a leader in local government, state government and federal government," she said on ABC's Good Morning America on Monday, January 21.
But is that enough to beat Trump?
- Elizabeth Warren
The Massachusetts senator is adored by the liberal intelligentsia in Washington and in New York but does Elizabeth Warren have the appeal to win back the millions of "Trump Democrats" in traditional blue states who defected in 2016?
Originally from Oklahoma and a registered Republican until the mid-1990s, Warren spent most of her life teaching law and specialising in bankruptcy protection in Texas and Pennsylvania as well as at Harvard.
She comes across as a patrician Ivy League figure, very remote from the ordinary American.
Trump certainly seems to be relishing taking her on.
"She'll be wonderful. I hope she maybe gets the nomination. That would be a wonderful thing for me," Trump said when Warren threw her hat in the ring.
"I've spent my career getting to the bottom of why America's promise works for some families, but others, who work just as hard, slip through the cracks into disaster and what I've found is terrifying. These aren't cracks that families are falling into, they're traps," Warren, 69, said when she launched her campaign for the nomination last month.
After the 2008 financial crisis she was recruited by Senate Majority Leader Harry Reid to help draft a US$700 billion bailout of the financial markets and she proposed the creation of a Consumer Financial Protection Bureau.
She became a senator in 2013 and has worked assiduously in Congress but she comes over as dour and patronising, compared to the sassy and simplistic president.
Trump has already begun baiting her.
Earlier this month, at an event for war veterans at the White House, he referred to her as Pocahontas because she has some Native American heritage several generations back.
"It is deeply unfortunate that the president of the US cannot even make it through a ceremony honouring these heroes without having to throw out a racial slur," Warren replied.
- Julian Castro
Castro is clearly basing going for the Hispanic vote, and it is huge nowadays.
"When my grandmother got here almost a hundred years ago, I'm sure she never could have imagined that just two generations later, one of her grandsons would be serving as a member of the United States Congress and the other would be standing with you here today to say these words: I am a candidate for President of the United States of America," Castro said when he launched his campaign for the nomination on 13 January.
Castro was raised primarily by his grandmother and excelled at school, attending Stanford University and Harvard Law School.
He returned to San Antonio, Texas, and became the city's mayor before Obama picked him to be Secretary of Housing and Urban Development in 2014.
"I am not a frontrunner in this race, but I have not been a frontrunner at any time in my life," said Castro, who is unlikely to last long in the primaries.
- Tulsi Gabbard
A member of the House of Representatives from Hawaii, Gabbard is a veteran of the 2003 Iraq War — having served in the National Guard — and sits on the House Foreign Affairs Committee.
She is the first American Samoan member of Congress and also the first Hindu on Capitol Hill, something that may prove difficult to sell in the "Bible belt".
In 2012 the Republicans struggled to sell Mitt Romney — a Mormon — to some of their more devout voters, but at least he was a Christian.
Gabbard is something of a maverick and has come out in favour of an anti-interventionist foreign policy.
In 2017 she met Syrian President Bashar al-Assad despite criticism from many fellow Democrats.
"When the opportunity arose to meet with him, I did so because I felt it's important that if we profess to truly care about the Syrian people, about their suffering, then we've got to be able to meet with anyone that we need to if there is a possibility that we could achieve peace, and that's exactly what we talked about," said Gabbard.
- John Delaney
Another virtual unknown outside of the Beltway, Delaney is a former member of the House of Representatives from Maryland.
The 55-year-old former congressman was the first out of the traps when he announced his plan to run way back in 2017.
Delaney said he was stepping down from Congress and bypassing a run for state governor in 2018 to concentrate on his bid for winning the Democratic nomination.
A self-made businessman, he will not want for money for his campaign but some of his positions put him out of touch with traditional Democrat voters — he wanted businesses to be able repatriate money earned overseas without paying taxes in exchange for buying infrastructure bonds.
- Kirsten Gillibrand
"I am not afraid of him," Gillibrand said of Trump, when she announced on 16 January she was setting up an exploratory committee, the first step on the road to running for the Democratic nomination.
Gillibrand, who was elected for her second full term in the Senate in November, rose to national prominence in recent years as a forceful proponent of the #MeToo movement.
She has said she wants to make her campaign about gender issues, but it is difficult how she can win the nomination on that single issue, let alone win the White House.
- Richard Ojeda
Ojeda, a former US Army paratrooper who has retained a buzzcut hair style, voted for Trump in 2016, but has since flip-flopped and now wants to be the man who turfs him out of office.
Trump called him "a total whacko" when Ojeda ran unsuccessfully as a Democrat in a congressional race in West Virginia in 2018.
"I relate to the people far more than what the President can ever relate to these people. The very people he comes down to West Virginia and stands in front of could never afford one single round of golf in some of his fancy country clubs. That's not where I stand," said Ojeda, when he announced his candidature.
- Andrew Yang
A businessman from New York, little is known about him, other than he wants to institute a universal basic income — a policy which was proposed by Bernie Sanders during the 2016 campaign.
THE POSSIBLE RUNNERS
- Joe Biden
He is taking his sweet time about it, but the former Vice President, who served under Barack Obama for eight years, is certainly mulling over a run at the top job.
"I'm the most qualified person in the country to be president. The issues that we face as a country today are the issues that have been in my wheelhouse, that I've worked on my whole life", he said modestly at a rally in Montana last month.
On the downside are his age — he would be 78 a few days after taking office — and his links to a highly discredited Washington establishment and an Obama administration which is not remembered fondly outside a Democrat hardcore.
- Beto O'Rourke
Despite the fact that he LOST a congressional race against Ted Cruz in last year's mid-term elections, the charismatic O'Rourke continues to be mentioned as a front-runner for the Democratic nomination.
Maybe the party machine thinks he has what it takes to be a gallant loser.
His supporters point to the fact that O'Rourke raised more money than any candidate for a Senate seat in American history-three times as much as Cruz, used social media brilliantly, inspired young people and ultimately lost by only 215,000 votes in a giant state like Texas.
But he still lost.
- Eric Garcetti
Garcetti is currently the Mayor of Los Angeles and a 50-1 shot with several US bookmakers to be the next occupant of the White House.
If you think you've heard the name Garcetti before somewhere, you're right.
His father Gil Garcetti was the Los Angeles district attorney who tried and failed to convict actor and former American football star O.J. Simpson for the murder of his wife Nicole and her friend Ronald Goldman in 1994.
"I am thinking about this. The majority of time goes to my work as mayor of Los Angeles, but everyone should think about what our role is in these difficult times, in these dangerous times," Garcetti said last year, in fluent Spanish, on the Mexican TV channel Univision.
Garcetti would certainly appeal to Hispanic voters, who are expected to make up around 15 percent of the total by 2020.
- Amy Klobuchar
The Minnesota senator might appeal to Midwestern voters who do not find candidates from California, New York or Massachusetts to their tastes.
"It is really important that whoever we put up has an understanding of those voices that weren't heard in 2016 from the Midwest and really from the middle of the country, and from the middle of our citizenry," Klobuchar said recently, when she said she was "considering" a tilt in 2020.
But the omens are not good for a Minnesota President.
The state produced Hubert Humphrey, who ran for President in 1968, and Walter Mondale, who was in line for the White House in 1984.
Both lost.
THE DEFINITE NON-RUNNERS
- Hillary Clinton
Yes, her again.
Despite her appalling campaign in 2016 — when she failed to come up with any policies and simply assumed she would win because she was a) the first woman to be nominated and b) she wasn't Donald Trump — many of her supporters were urging her to consider another shot at Trump next year.
But her advisers said in October she was not planning to run again.
Whether she endorses one of the nominees remains to be seen, but it could be the kiss of death for them.
- Oprah Winfrey
In January 2018 Oprah Winfrey ruled herself out of the running.
"I've always felt very secure and confident with myself in knowing what I could do and what I could not. And so it's not something that interests me. I don't have the DNA for it," Winfrey told InStyle magazine.