New Jersey Sen. Cory Booker has been one of the most active Democrats being talked up as a potential 2020 challenger against Trump.
Booker has already made stops in Iowa, New Hampshire, and South Carolina — all key early voting states when the 2020 primaries get underway. Booker also campaigned alongside prominent Democrats in the lead up to the 2018 midterm elections, showing that he is cultivating support among the party's base.
After Texas Rep. Beto O'Rourke lost his long shot Senate campaign to unseat incumbent Republican Ted Cruz, he quickly pivoted to considering future ambitions.
Just a couple weeks after the 2020 midterm elections, O'Rourke began receiving invites to engage voters in New Hampshire and Iowa.
O'Rourke, who showed unparalleled fundraising prowess in 2018, also acknowledged less than two weeks after losing the Senate bid that he was mulling another run at something and was keeping his options on the table.
"Now that [unseating Cruz] is no longer possible, we're thinking through a number of things and Amy and I made a decision not to rule anything out," he said at a town hall in El Paso.
Vermont Sen. Bernie Sanders was the underdog in 2016 as he was the only Democrat to mount a challenge against Hillary Clinton. Heading into 2020, Sanders is backed by significant polling results showing him as a top choice among Democrats.
Sanders, 77, has already traversed the early primary states in the past year and released a book — all tell-tale signs he could be gearing up for another run.
Julian Castro, who served as the Secretary of Housing and Urban Development during the Obama administration, has already launched an presidential exploratory committee.
In an interview with the Associated Press, Castro suggested he was heavily leaning toward mounting a 2020 presidential bid.
"People might say right now, 'Well, hey, you’re way down here in polling that’s taken.' The most dangerous place to be right now is actually in the pole position," he told the AP. "It doesn’t bother me that in December of 2018 I’m not right up at the top of the list. If I decide to run, it would be because I believe I have a compelling message and I’m going to work hard and get to the voters and I believe I can be successful."
Castro, who was also the mayor of San Antonio from 2009-2014, said he would make a final decision in early January.
Massachusetts Sen. Elizabeth Warren hinted that a 2020 presidential run was in the cards when she said she would mull over the decision after the 2018 midterm elections concluded.
While Warren already has a large war chest of campaign finances, she has run into some problems. Most of her constituents do not want her to pursue the presidency, which a Boston Globe editorial hinted at earlier this month.
Nevertheless, Warren has continued to make moves, including releasing the results of a DNA test to show that she has Native American ancestry in an attempt to quell ridicule from President Trump. The backlash to the DNA test stunt has prompted internal concerns about her standing in a crowded 2020 field, according to the New York Times.
Billionaire business mogul and former New York City Mayor Michael Bloomberg has been flirting with a 2020 presidential bid for some time, going as far as to change his political affiliation to join the Democratic Party.
Bloomberg has also made the trips to Iowa and New Hampshire in recent months, which draws the attention of political journalists everywhere.
During a December appearance on ABC's "The View," Bloomberg said, "I think most Democrats want a middle-of-the-road strategy."
"They want to make progress, but they’re not willing to go and to push something that has no chance of ever getting done and wasting all their energy on that," he said.
Unlike the rest of the Democrats on this list, Maryland Rep. John Delaney has already been actively campaigning since June 2017, when he announced he would spend several years running for president and forgoing re-election to his House seat.
In that time, Delaney has made 20 visits to Iowa and almost a dozen trips to New Hampshire. Delaney has also managed to boost his previously unknown name recognition in early voting states, bringing up his name ID among Iowa voters.
But Delaney has a long road ahead, especially as more high-profile Democrats begin to enter the 2020 race.
California Rep. Eric Swalwell, one of the more outspoken members of the House Intelligence Committee, has been making moves signaling a 2020 presidential run.
Like other eager contenders, Swalwell has ventured to Iowa, New Hampshire, and South Carolina in recent months. Swalwell is also returning to Iowa in the days before Christmas to participate in a town hall about gun violence.
Swalwell, 37, was also the subject of a report stating he was 100% committed to a 2020 run, but quashed such rumors saying he had nothing to announce "yet."
Former Vice President Joe Biden is frequently listed among the top contenders for the 202 presidential nomination.
Biden, 76, was active in campaigning alongside Democratic candidates in the 2018 midterm election cycle, attending numerous get out the vote rallies across the United States.
Recently, Biden upped the ante when he said he is 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 added.
And you can expect a decision from Biden as early as January, according to the Associated Press.