"This is a multilevel chess game with more candidates than anyone has seen," said Joe Trippi, a Democratic strategist.
"I'm not sure anyone knows what the rules are, much less how to get through the marathon that started today."
This collection of candidates will be emerging as the Democratic Party is trying to determine how to define itself in the era of Trump.
Questions abound: Should the party fight for the white working-class voters who flocked to Trump? Or should they look to younger, nonwhite voters whose numbers are growing faster than their voting habits? Should they tangle with Trump? Should they rise above him? Is a fresh face needed, or an experienced hand?
"This is an important time for Democrats to discuss what we're all about. Where do we stand as a party?" said Andrea Steele, the founder of Emerge America, an organisation that encourages women to run for office.
"We're in this defining moment. We have this president in there. And everything is at stake. Everything."
Political observers are used to thinking about presidential candidates as occupying defined lanes. But the 2020 race will include several candidates for some lanes, creating mini-primaries among the more crowded sections of the party.
The challenge for candidates will be to win in their lanes while expanding beyond that niche to forge alliances with other groups.
Additional candidates are expected to make their intentions known in the next few weeks, turning the opening days of 2019 into a drumbeat of potential challengers to Trump.
Warren's opening salvo, broadcast via an emailed announcement video, defined her as an economic populist ready to stand up to Trump.
Her slogan made her attitude clear: "Join the Fight".
"She is a very strong advocate — an edgy advocate — and is that what people are going to be looking for in the era of Trump? Do you fight edgy with edgy?" asked David Axelrod, the chief strategist for Barack Obama's presidential campaigns.
"She has a very distinctive point of view that is very much born out of her life's work," he added. "And she was well known before she was in politics."
Warren also has a structural advantage over other candidates, Axelrod said.
"She was the most rigorous about being in touch with and supporting primary candidates around the country (during the Midterm campaigns). She's clearly been very methodical."
And though she tripped up with a poorly received release of a DNA test intended to prove that her family stories about Native American ancestry were accurate, Axelrod said the controversy has been overstated.
"Presidential campaigns are filled with high moments and low moments, and moments where people are second-guessing you," he said.
Trump revived his criticism of Warren's claimed ancestry in a New Year's interview with Fox News, adding that "I hope she does well; I'd love to run against her".
Asked whether she could win, he replied: "Well, that I don't know, you'd have to ask her psychiatrist".
Warren's announcement was expected, but the continuing tension over the line-up centres on three men whose decisions whether to run could influence other challengers — former Vice-President Joe Biden, Senator Bernie Sanders, I, and Beto O'Rourke, the Democratic congressman from El Paso, Texas, who left his seat to unsuccessfully challenge Republican incumbent Ted Cruz for a Senate seat.
Biden would have a historic advantage: In modern history, every former vice-president who has sought his party's nomination has clinched it.
The best-known candidate in the race, Biden has run and lost two previous campaigns for the Democrats' presidential nomination.
He said in November that he would not announce whether he would run until the new year. Asked whether that meant a January announcement, he said: "I wouldn't announce if I were going to run that early. It would be too early to start it."
Sanders, though unsuccessful in 2016, caught fire with his bold message and grumpy authenticity. He arguably has the most to lose from Warren's entry into the race, as they offer similar economic populist messages and reside in the same region.
Sanders tweeted several hours after Warren's announcement, in what appeared to be a bid to stay in the conversation.
"We must have the courage to take on the greed and ideology of the billionaire class and to fight for a world of economic, social, racial and environmental justice," said Sanders.
"Will this be an easy struggle? Certainly not."
O'Rourke gained national attention by raising more than US$70 million, the largest Senate haul in history, in his bid to topple Cruz.
A video O'Rourke tweeted on Saturday questioning the wisdom of building a border wall was viewed more than five million times in four days.
Other candidates occupy loose niches, often overlapping ones.
The 2020 contest may also set a record for female and non-white candidates, their energy fuelled by the party's successes in the midterm elections.
"The goal of 2020 will be about inspiring and turning out voters who have never participated," said Rashad Robinson, executive director of Colour of Change, a racial justice organisation. "This will not be about watering down a message. This cannot just be about defeating Trump."
Senators Kamala Harris and Cory Booker, both black, have proved themselves popular in early voting states, some of which are dominated by non-white voters.
Harris took to Twitter after Warren's announcement to offer a recap of her 2018 accomplishments.
She also reprised a tweet that she'd sent earlier in the year: "My advice to Black girls everywhere: whenever you find yourself in a room where there aren't a lot of people who look like you . . . remember that you have an entire community in that room with you, all of us cheering you on."
One of the two Democrats who have officially announced their candidacy, former San Antonio Mayor Julian Castro, also is expected to seek the party's young and minority voters. The other official entrant is John Delaney, a Maryland congressman who is retiring from the House to pursue his bid.
No candidate will get far without building an organisation. That's were another division lies, between the Democrats who have already put together their teams and Democrats still thinking about whether to run.
Booker, Harris, senators Kirsten Gillibrand of New York and Jeff Merkley of Oregon, and Colorado Governor John Hickenlooper are in the first camp.
All of them have signalled who would lead their campaigns, and in some cases, where their headquarters will be located; all have made overtures to potential staffers in early primary states and have begun making arguments about how they could win.
Other Democratic contenders are not so far along, thinking in public and in private about whether they can find a path.
Senators Amy Klobuchar of Minnesota and Sherrod Brown of Ohio acknowledge that they're looking at 2020, often by saying that the idea has been put to them by supporters who want a candidate who can win.
The complicated aspect of the upcoming season is that many of these potential candidates have overlapping strengths.
Biden, Brown and Klobuchar are thought to have appeal in the Upper Midwest, the home of the many voters who abandoned the Democratic Party for Trump in 2016.
Warren, Harris, Klobuchar and Gillibrand are all women.
The electorate might also have contradictory desires.
"Everyone likes a fresh face, but experience matters," said Randi Weingarten, the president of the 1.7 million-member American Federation of Teachers.
All of the candidates will soon be making visits to early states, if they haven't already.
Warren plans to visit Iowa this weekend, pending votes in the Senate. She hasn't been there since 2014, when she campaigned for Democratic US Senate candidate Bruce Braley, who lost to Republican Joni Ernst.
She called supporters and Democratic VIPs, including Tom Vilsack, the former Iowa governor and agriculture secretary under Obama.
He called Warren a "welcome addition" to the field.
But he also had a warning to all: "You really have to be 1000 per cent committed to this. It's an incredible grind. And incredible set of pressure-packed days. You have to understand it's going to be the most difficult thing you do in your life."
