Here are 43 possible candidates who could take on Trump in 2020:
Former Vice President Joe Biden: Biden, 74, said he "regretted" not running in 2016. He stoked major speculation about 2020 with a busy travel schedule but later said, "Guys, I'm not running!"
Sen. Bernie Sanders (I-Vt.): Sanders, 75, emerged as a leader on the left after his 2016 presidential run, and he's working with the Democratic National Committee to help unite the party. He wouldn't rule out a 2020 run but said in January it's "much too early" to discuss another bid.
Sen. Elizabeth Warren (D-Mass.): Warren, 67, has become one of the biggest thorns in Trump's side. In an April interview, Warren said she has no plans to run in 2020 and is focused on her 2018 reelection.
Sen. Kamala Harris (D-Calif.): Harris, 52, has been in the Senate for just four months, but the rising star is already floated as a potential contender. The former California attorney general said she's not thinking about future campaigns.
Sen. Chris Murphy (D-Conn.): The vocal gun control advocate has been another strong Trump critic. The White House reportedly asked consultants to look into Murphy, 43, and four other possible Trump challengers.
Sen. Amy Klobuchar (D-Minn.): Klobuchar, 56, is running for reelection next year, but she stoked major speculation with her plans to travel to Iowa, a crucial state on the primary schedule, this weekend.
Facebook CEO and founder Mark Zuckerberg: Zuckerberg, 32, who also co-founded an immigration advocacy organization, created some buzz when he said he'll visit all 50 states this year.
Sen. Cory Booker (D-N.J.): Booker's meteoric rise from a mayor of Newark, N.J., to U.S. senator has prompted speculation about a future run for president. While Booker, 48, won't discuss future plans, he didn't rule it out, either.
Sen. Kirsten Gillibrand (D-N.Y.): Gillibrand, Clinton's Senate successor in 2009, earned Democratic cred by leading the Senate Democrats in votes against confirming Trump's Cabinet nominees. But Gillibrand, 50, says she's focused on her 2018 reelection campaign and recently ruled out a 2020 run.
Colorado Gov. John Hickenlooper: Hickenlooper, 65, who was on Clinton's vice presidential shortlist, has been floated. He told the Denver Post that he's not running for president.
New York Gov. Andrew Cuomo: Cuomo, 59, is running for reelection in 2018, but he hired two fundraisers from Florida, a sign that he could be considering a presidential run.
Former Maryland Gov. Martin O'Malley: O'Malley, 54, didn't gain much traction in his 2016 run, but he's already testing the waters again. A political action committee affiliated with him polled Democratic caucus voters in Iowa, and he visited New Hampshire in April.
Former Housing and Urban Development Secretary Juli n Castro: Castro, 42, was on Clinton's vice presidential shortlist. The former San Antonio mayor drew national attention for his 2012 Democratic National Convention speech.
Sen. Tim Kaine (D-Va.): Kaine, 59, grew his profile as Clinton's running mate. After the election, Kaine ruled out running for president or vice president in 2020. He's up for reelection in 2018.
Sen. Al Franken (D-Minn.): Franken, 65, emerged as a tough critic during the confirmation hearings for Trump's Cabinet picks. In an interview, he said he's not running, noting that senators generally don't fare well running for president.
Sen. Sherrod Brown (D-Ohio): Brown, 64, was on Clinton's running mate shortlist. He could face a tough reelection in 2018, though, after Trump won his state in 2016.
Former Massachusetts Gov. Deval Patrick: Patrick, 60, left office in 2015 and now works at Bain Capital. He's been previously floated as a presidential contender and is close with a former top Obama adviser, David Axelrod.
New York City Mayor Bill de Blasio: De Blasio, 55, is running for reelection this year. His political prospects have been buoyed by the news that he won't face charges in a federal investigation into his 2013 campaign fundraising.
Dallas Mavericks owner Mark Cuban: The billionaire businessman and "Shark Tank" star frequently clashed with Trump in 2016 and endorsed Clinton. Cuban, 58, has said "we will see" about whether he runs for president.
Environmental activist Tom Steyer: The billionaire donor, 59, who runs a climate change advocacy group, is considered a possible candidate for California governor in 2018.
Democratic National Committee Chairman Tom Perez: The former Labor secretary, 55, was elected head of the national party this year and is looking to rebuild after the 2016 elections.
Minnesota Gov. Mark Dayton: Dayton, 70, was recently diagnosed with prostate cancer but plans to finish out his term, which ends in 2018.
Virginia Gov. Terry McAuliffe: The term-limited governor will be out of office in January 2018. McAuliffe, 60, is a well-connected ally of both Bill and Hillary Clinton and a former DNC chairman.
California Lt. Gov. Gavin Newsom: Newsom, 49, launched an early 2018 bid for California governor. He's said being president seems "like the most miserable job in the world."
Facebook COO Sheryl Sandberg: Sandberg, 47, drew praise for her book "Lean In," which discusses women in the workforce, but she has said she won't run and will "continue to say no."
Starbucks CEO Howard Schultz: Schultz, 63, is stepping down from his role and will be executive chairman. He was urged by friend to run in 2016, but he endorsed Clinton.
Former first lady Michelle Obama: The former first lady proved a formidable campaigner for Clinton in 2016, but Obama, 53, and others close to her have said she won't run for elected office.
Dwayne "The Rock" Johnson: The wrestler and star of the "Fast and the Furious" film franchise has flirted with running for office. A registered Republican, Johnson, 45, spoke at the party's convention in 2000, but documentary filmmaker Michael Moore has urged him to run. One potential political ally: Warren, who has described herself as a fan of Johnson's HBO show "Ballers."
Rep. Tulsi Gabbard (D-Hawaii): Gabbard, 36, hasn't been afraid to buck her party. She drew scrutiny for secretly meeting with Syrian President Bashar Assad and criticizing the U.S. strike on Syria following Assad's alleged use of chemical weapons on civilians.
Rep. Keith Ellison (D-Minn.): Ellison, 53, was a prominent Sanders supporter and was a leading contender for DNC chairman before losing to Perez. He now serves as the DNC's deputy chairman.
Rep. Sean Patrick Maloney (D-N.Y.): Maloney, 50, considered a bid to lead the House Democrats' campaign arm this cycle, but passed. He wrote Democrats' autopsy on the 2016 elections.
California Gov. Jerry Brown: Brown, whose term is up in 2018, doesn't think he'll run for office again, but wouldn't rule it out. Brown, 79, has run for president three times.
Media mogul Oprah Winfrey: Winfrey, 63, who endorsed Clinton, is frequently floated for president but has said she will "never" run.
Former Sen. Russ Feingold (D-Wis.): Feingold, 64, served in the Senate from 1993 to 2011, but he lost a comeback bid in 2016. He considered a presidential run in 2008.
Former Vermont Gov. Howard Dean: Dean, 68, unsuccessfully ran for president in 2004. Dean, whose tenure as DNC chairman from 2005 to 2009 was marked by its successful "50 States Strategy," briefly ran for DNC chair this year before dropping out.
Former Vice President Al Gore: Gore, 69, who lost the 2000 presidential election after a Supreme Court decision, reemerged in politics when stumping for Clinton last year.
Sen. Mark Warner (D-Va.): Warner, 62, ruled out a 2020 run: "I think that window is probably shut."
Former Sen. Jim Webb (D-Va.): Webb ran for the Democratic nomination in 2016 but dropped out after only polling in the single digits. He briefly weighed running as an independent. Since the 2016 race, Webb, 71, has pitched himself as a politician who can understand the white working-class voters who are flocking to Trump.
Los Angeles Mayor Eric Garcetti: Garcetti, 46, is considering a 2018 bid for California governor, but The New York Times reported that national donors have urged him to run for president.
Rep. Seth Moulton (D-Mass.): Moulton, an Iraq War veteran, was also mentioned in the New York Times story and privately says he's not ruling out a bid. Moulton, 38, brushed aside the story but is fundraising off of it anyway.
New Orleans Mayor Mitch Landrieu: Landrieu, 56, called the a New York Times story about him considering a run "hysterical." He gained notoriety after defending the removal of Confederate memorials in New Orleans.
Sen. Tammy Duckworth (D-Ill.): Duckworth, 49, recently made the switch from the House to the Senate. She's an Iraq War veteran and lost both of her legs while serving as a Army helicopter pilot during the war.
Washington Gov. Jay Inslee: The two-term governor was a congressman for more than a decade. Inslee, 66, played a role in his state's lawsuit against Trump's travel ban.
