Republican President Donald Trump officially launched his 2020 campaign and is expected to clinch the GOP nomination, but the field is still wide open for Democrats and independent candidates hoping to run against him. For the first debate, 20 candidates have met the threshold of polling and fundraising support necesary to participate. For later debates, the qualifications will get stricter.
VIDEOS: Meet the 2020 Democratic candidates
Here's an alphabetical list of the candidates who made it in.
Sen. Michael Bennet
The congressman from Colorado is considered a rising star within the party who paints himself as a "pragmatic, independent thinker" willing to reach across the aisle. He notably came into the spotlight earlier this year with his dramatic cricitism of Sen. Ted Cruz during the government shutdown. In April, Bennet announced he had been diangosed with prostate cancer. His treatments delayed his candidacy, but he ultimately entered the race in May.
Former Vice President Joe Biden
After months of speculation, former Vice President Joe Biden officially joined those vying to become the Democratic candidate for president in 2020 on April 25, 2019.
Biden enters the race as a frontrunner, though he faces questions about whether he is now too moderate to fit the party and about his age. If Biden runs against Trump in the general election, either would be the oldest president ever elected.
SEE ALSO: What to know about former VP, Senator Joe Biden
Sen. Cory Booker
Sen. Cory Booker officially launched his campaign on Feb. 1, the first day of Black History Month. There was speculation Booker would run, as he was among the Democrats who were already testing their ability to connect with swing state voters, visiting Iowa, New Hampshire and South Carolina to build connections with key powerbrokers. Notably, Booker paced the stage at an Iowa Democratic Party fall banquet, repeatedly quoting Martin Luther King in a sermon-like speech to 1,200 of the state's most influential party activists, officials and donors in October.
RELATED: Things to know about presidential candidate Cory Booker
Pete Buttigieg, mayor of South Bend, Indiana
Democrat Pete Buttigieg, the 37-year-old mayor of South Bend, Indiana, is the youngest of the 2020 candidates. When he enetered the race, Buttigieg has touted his work to improve his city of 100,000 residents as he's prepared for a jump from local politics to a presidential campaign. He's also said Democrats could benefit from a new generation of leaders as they try to unseat Trump in 2020.
Buttigieg is a Rhodes scholar who was first elected mayor of his hometown in 2011 at age 29, making him the youngest mayor of a U.S. city with at least 100,000 residents. A lieutenant in the Navy Reserve, he served a tour in Afghanistan in 2014.
Buttigieg would be the first openly gay nominee of a major presidential party; he married his husband, Chasten, last year. He would be the first mayor to go directly to the White House. And he would be the youngest person to become president, turning 39 the day before the next inauguration, on Jan. 20, 2021.
The mayor is taking the debate stage as he faces a divided community back home after a police shooting. On June 16, a white police officer in South Bend fatally shot a black man after he was allegedly caught breaking into cars and allegedly approached the officer with a kinfe. Over the weekend, Buttigieg took time off the campaign trail to attend a protest and march.
Former Secretary of Housing and Urban Development Julian Castro
Former Obama administration housing chief Julian Castro officially kicked off his presidential campaign on Jan. 12.
Castro, the Democratic former mayor of San Antonio who served in President Barack Obama's second term, told Nevada Democrats Jan. 8 that as a presidential candidate, he's going to be talking about improving public education, equal treatment under the criminal justice system and addressing what he called the existential threat of climate change.
Bill de Blasio, mayor of New York City
Shortly before an expected announcement on Good Morning America, New York City mayor Bill de Blasio posted a campaign on YouTube. A Quinnipiac University poll in April found that 76 percent of New Yorkers did not think de Blasio should run for president.
Former Rep. John Delaney
The three-term Maryland legislator announced his intention to run for the presidency back in 2017 and has since run multiple television ads in battleground Iowa. He visited each of Iowa's 99 counties in 2018, according to the AP.
Despite the nationalization of more recent presidential campaigns, Delaney has been taking a chapter from previous Iowa Cinderella stories. As a little-known Georgia governor, Jimmy Carter famously crisscrossed Iowa to emerge as a rising star in 1976.
Rep. Tulsi Gabbard
U.S. Rep. Tulsi Gabbard, a Hawaii Democrat known for bucking the party establishment and for criticizing U.S. foreign policy in the Middle East, formally launched her campaign for president in January.
The 38-year-old Iraq War veteran is the first Hindu elected to Congress and the first member born in the U.S. territory of American Samoa.
Sen. Kirsten Gillibrand
New York Sen. Kirsten Gillibrand, who was appointed to the Senate in 2009 to fill the seat vacated by Hillary Clinton, has been among the Senate's most vocal members on issues like sexual harassment, military sexual assault, equal pay for women and family leave, issues that could be central to her presidential campaign.
RELATED: What to know about 2020 candidate, NY Sen. Kirsten Gillibrand
As she works to distinguish herself from likely rivals, Gillibrand will be able to draw from the more than $10.5 million left over from her 2018 re-election campaign that she can use toward a presidential run.
Sen. Kamala Harris
Sen. Kamala Harris (D-CA) officially launched her run by making the announcement on Good Morning America on Martin Luther King Day.
"The American people deserve to have someone who is going to fight for them...and put them in front of self-interest," she said.
Harris has served as a senator since 2017 after previously serving as the district attorney of San Francisco and California's attorney general.
RELATED: Things to know about presidential candidate Kamala Harris
Former Gov. John Hickenlooper
The former governor of Colorado became the second governor to enter the race on March 4.
Hickenlooper, who has in the past prided himself for staying above partisan fights, painted himself as a uniter in a campaign video announcing his run.
Gov. Jay Inslee
The governor of Washington officially joined the race on March 1, 2019. Ahead of a public announcement in Seattle, Inslee launched his campaign with a video about the urgency of defeating climate change.
VIDEO: This is our moment, our climate, our mission — together, we can defeat climate change. That's why I'm running for president. Join #OurClimateMoment today https://t.co/zg8ILGyk0Z pic.twitter.com/pUZVxyzfc5— Jay Inslee (@JayInslee) March 1, 2019
Inslee, who has aggressively critiqued President Trump, was the first governor to join the Democratic primary in a crowded race.
Sen. Amy Klobuchar
Amy Klobuchar, a three-term senator from Minnesota, kicked off her campaign Feb. 10 in Minneapolis. She's the most prominent Midwesterner in the race so far and has drawn support from voters in urban, suburban and rural areas, including in dozens of counties Trump won in 2016.
She has said that success could translate to other Midwestern states such as Michigan and Wisconsin, reliably Democratic in presidential races for decades until Trump's victory over Hillary Clinton.
Former U.S. Rep. Beto O'Rourke
After a surprisingly close loss to Republican Sen. Ted Cruz for his Senate seat, O'Rourke is after the White House. In lieu of a produced campaign video, the announcement was made in a video released early on March 14 of the candidate speaking to the camera with his wife by his side. The outgoing Texas congressman called for "the greatest grassroots campaign this country has ever seen."
RELATED: What to know about Beto O'Rourke
Ohio Rep. Tim Ryan
Ohio Rep. Tim Ryan jumped into the 2020 presidential race on April 4, portraying himself as a candidate who can bridge Democrats' progressive and working-class wings to win the White House.
Ryan, 45, announced his primary bid on ABC's "The View." The congressman resisted being labeled a political centrist by the talk show's hosts, who pointed out that he's a recreational hunter with past backing from the National Rifle Association. In 2015, he reversed his past opposition to abortion in favor of abortion rights.
"I'm a progressive who knows how to talk to working-class people, and I know how to get elected in working-class districts, because, at the end of the day, the progressive agenda is what's best for working families," Ryan said.
Sen. Bernie Sanders
2016 presidential contender Sen. Bernie Sanders will be joining the race again, he announced Feb. 19. Sanders, who was active on the midterm campaign trail as he endorsed liberal candidates around the country, emerged as one of the early favorites in the opening phase of the 2020 campaign even before his announcement.
Calif. Rep. Eric Swalwell
California Rep. Eric Swalwell entered the race April 8 during an appearance on "The Late Show with Stephen Colbert."
The congressman said, "I see a country in quicksand, unable to solve threats from abroad, unable to make life better for people here at home."
He added, "None of that is going to change until we get a leader who is willing to go big on the issues we take on, be bold in the solutions we offer and do good in the way that we govern."
The 38-year-old Iowa native was elected in 2012 to represent California's 15th Congressional District.
Sen. Elizabeth Warren
After announcing an exploratory committee on New Year's Eve, Sen. Elizabeth Warren formalized her bid for the presidency at a Feb. 9 rally in Lawrence, Massachusetts.
She's banking on a populist call to fight economic inequality, which she hopes will distinguish her and help her move past the controversy surrounding her past claims to Native American heritage.
Warren has spent the past decade in the national spotlight, first emerging as a consumer activist during the financial crisis.
Author and activist Marianne Williamson
Spiritual teacher and author Marianne Williamson announced her campaign for the presidency on Jan. 28.
"I want to engage voters in a more meaningful conversation about America," Williamson said in a statement obtained by ABC News. "About our history, about how each of us fit into it, and how to create a sustainable future. Our national challenges are deep, but our political conversation is shallow."
Williamson is known for her 1992 bestseller "A Return to Love" and her appearances on "The Oprah Winfrey Show." She founded Project Angel Food, a charity that delivered meals to home-bound patients battling AIDS and other diseases, and has also worked to support peacekeeping causes.
Entrepreneur Andrew Yang
Andrew Yang filed a statement of candidacy in 2017. Yang is the founder of Venture for America, an organization focused on job creation in several cities across America. His campaign platform is focused on creating an "America of opportunity, freedom, equality, and abundance."
The Associated Press and ABC News contributed to this report.