Who has dropped out?
So far 18 candidates have dropped out of the race, with more likely to end their campaigns as the top tier candidates continue to increase their lead.
Here's a look at who has dropped out of the race:
Cory Booker
Cory Booker has served as a US senator from New Jersey - the first African American in the state's history to hold the office since 2013. He was the mayor of Newark from 2006 to 2013.
After struggling in the polls and fundraising, Booker dropped out of the race in January.
"It's with a full heart that I share this news - I'm suspending my campaign for president," he tweeted when announcing he was ending his bid.
"To my team, supporters, and everyone who gave me a shot - thank you. I am so proud of what we built, and I feel nothing but faith in what we can accomplish together," he added.
Booker had qualified for five debates, but failed to meet the requirements for the sixth event.
Steven Bullock
Montana Governor Steve Bullock quit his bid for the Democratic nomination in early December.
"Today, I am suspending my campaign to become the Democratic Party's nominee for president," Bullock, 53, said in a statement on Monday. "While there were many obstacles we could not have anticipated when entering this race, it has become clear that in this moment, I won't be able to break through to the top tier of this still-crowded field of candidates."
Bullock, 53, will return to Montana to finish out his second term as governor. He will not run for the US Senate, despite encouragement from party leaders to do so and help Democrats wrest control of Congress's upper house from Republicans.
Bullock was re-elected in conservative Montana in 2016, making him the only Democratic presidential contender who had won a statewide election in a state Trump carried in 2016.
Bullock failed to make the presidential debate stage in June, but did so in July. Due to the stricter guidelines for the events in September, October and November, he did not qualify.
Julian Castro
Julian Castro was elected mayor of San Antonio, Texas in 2009 and served until 2014.
He served as the 16th US secretary of Housing and Urban Development (HUD) under US President Barack Obama from 2014 until 2017.
He ended his presidential bid in January, saying in a video: "I've determined that it simply isn't our time."
He said he wasn't done fighting. "I'll keep working towards a nation where everyone counts, a nation where everyone can get a good job, good health care and a decent place to live."
Castro made the debate stage for the first four events, but failed to qualify for the fifth and sixth debates.
Bill de Blasio
The New York City mayor emerged as a progressive standard-bearer in 2013, when he won the first of two four-year terms at the helm of the country's biggest city on a platform of addressing income inequality. But he has struggled amid middling approval ratings and some political setbacks to build a national profile.
After making the first two 2020 Democratic debates, but failing to qualify for the September event, the New York City mayor dropped out of the race in mid-September.
"I feel like I've contributed all that I can to this primary election," de Blasio told MSNBC when announcing he was withdrawing from the race. "It's clearly not my time."
John Delaney, 56
John Delaney served as a US congressman for Maryland's sixth district from 2013 to 2019.
He was the first Democrat to formally declare a run for the party's 2020 presidential nomination, but at the end of January he said he was dropping out of the race.
Delaney, who had already been campaigning for more than a year, announced the end of his presidential run in an interview on CNN.
He failed to qualify for the first debate, but was able to do so for the second. He could not, however, meet the guidelines for rest of the debates.
Kirsten Gillibrand
Kirsten Gillibrand has served as a US senator from New York since 2009.
After failing to qualify for the third Democratic presidential debate, Gillibrand, who campaigned on a platform centred on women's rights, dropped out of the race.
In announcing her decision on August 28, Gillibrand told US media she had not decided which candidate to endorse.
"I think that women have a unique ability to bring people together and heal this country," Gillibrand told the New York Times newspaper.
"I think a woman nominee would be inspiring and exciting," she added.
Mike Gravel
Mike Gravel, the 89-year-old former senator made a little-known run for the Democratic nomination in 2008, took another stab at it early in the Democratic race.
His goal was to make the debate stage, but when that didn't happen, he officially ended his campaign in August, and endorsed Sanders.
Kamala Harris
Kamala Harris has served as a US senator from California since 2017. In early December, she ended her bid for the nomination, saying her campaign doesn't have the financial resources to continue.
"I've taken stock and looked at this from every angle, and over the last few days have come to one of the hardest decisions of my life," Harris said in an email to supporters on Tuesday. "My campaign for president simply doesn't have the financial resources we need to continue."
Harris had qualified for all five debates that took place before she ended her bid.
John Hickenlooper
John Hickenlooper served as the governor of Colorado from 2011 to 2019.
He announced he was ending his presidential bid on August 15 in a video posted on Twitter.
"While this campaign didn't have the outcome we were hoping for, every moment has been worthwhile and I'm thankful to everyone who supported this campaign and our entire team," Hickenlooper tweeted.
Later in August, Hickenlooper announced he would run in the US Senate race against Republican incumbent Cory Gardner in Colorado.
Jay Inslee
Jay Inslee has served as the governor of the state of Washington since 2013.
On August 21, he announced he was dropping out of the race, saying "it has become clear that I'm not going to be carrying the ball. I am not going to be the president."
Inslee made fighting climate change the central issue of his campaign. In announcing his withdrawal, Inslee said he hopes other 2020 candidates would use his detailed 10-year climate plan.
Wayne Messam
Wayne Messam has served as mayor of Miramar, Florida, since 2015.
In mid-November, he tweeted that he was suspending his campaign.
Messam had failed to make a single debate stage during the campaign.
Seth Moulton
Seth Moulton has served as the US representative for Massachusetts's sixth congressional district since 2015.
On August 23, he announced he was dropping out of the 2020 race, telling US media if one of the more progressive candidates won the nomination it could make it harder for the Democrats to beat Trump.
"I think it's evident that this is now a three-way race between Biden, Warren and Sanders, and really it's a debate about how far left the party should go," Moulton told the New York Times newspaper.
Richard Ojeda
Richard Ojeda was the first official presidential contender to drop out of the race.
In January, the former West Virginia state senator announced he was suspending his campaign, acknowledging he "does not have the ability to compete".
Beto O'Rourke
Beto O'Rourke served Texas's 16th congressional district in the House of Representatives from 2013 to 2019.
O'Rourke gained fame last year for his record fundraising and ability to draw crowds before his unexpectedly narrow loss in the US Senate race against Republican incumbent Ted Cruz.
On November 1, O'Rourke announced he was dropping out of the presidential race. The New York Times reported that his campaign was facing financial strain.
"My service to the country will not be as a candidate or a nominee," O'Rourke said in an online post.
"Though it is difficult to accept, it is clear to me now that this campaign does not have the means to move forward successfully," he wrote on Medium. "Acknowledging this now is in the best interests of those in the campaign; it is in the best interests of this party as we seek to unify around a nominee, and it is in the best interests of the country."
Tim Ryan
Tim Ryan has served as a US House representative from Ohio's 13th district since 2003.
On October 24, he announced he is ending his bid for the nomination and would instead seek re-election in his House race.
Ryan's campaign was marked by slow fundraising and low poll numbers.
He qualified for the first two debates, but failed to do so for the September and October events.
Joe Sestak
Former US Representative Joe Sestak joined the race in June.
In announcing his candidacy, Sestak, a retired three-star US Navy admiral, emphasised his 31-year military career, the need to restore US leadership in the world and tackle challenges from climate change and China's growing global influence.
But after failing to qualify for any of the debates and impress donors, the 67-year-old dropped out of the race in early December.
In a statement posted on his Twitter page, Sestak offered his thanks to supporters "for the honor of running for president" as "I end our campaign together".
"Without the privilege of national press, it is unfair to ask others to husband their resolve and to sacrifice resources any longer," he wrote.
Eric Swalwell
Eric Swalwell, an Iowa native, has served as a House representative from California's 15th congressional district since 2013.
He dropped out of the presidential race after the first primary debate in June.
Marianne Williamson
Marianne Williamson is an author, entrepreneur and activist.
The self-help guru dropped out of the race in January.
"I stayed in the race to take advantage of every possible effort to share our message. With caucuses and primaries now about to begin, however, we will not be able to garner enough votes in the election to elevate our conversation any more than it is now," Williamson said in a statement.
Williamson qualified for the first two debates, but failed to do so in September, October, November and December.
SOURCE: AL JAZEERA AND NEWS AGENCIES