Three months into the Democratic presidential primary battle, the contest has emerged as one of the most wide open in a generation — with multiple candidates showing staying power, enthusiastic crowds at events and money pouring into campaign coffers.

The biggest remaining variable is former vice president Joe Biden, who is moving toward an expected announcement in late April, amid questions about his long record and ability to navigate the modern political landscape.

But the opening stretch has brought some clarity.

As part of the most diverse presidential field ever, Sen. Elizabeth Warren (Mass.) has become the policy pacesetter, while Sen. Kamala D. Harris (Calif.) has demonstrated star power. Both are building organizations prepared to engage in a long race.

South Bend Mayor Pete Buttigieg (D) spoke about looking ahead to the future and "picking up the pieces of reckless policies that are being made right now." (Jenny Starrs/The Washington Post)

One of the uncertainties as the year began — whether Sen. Bernie Sanders (Vt.) would be able to rekindle the excitement that greeted him in his 2016 campaign — has received a resounding reply with a solid group of supporters and impressive fundraising numbers. Many Democratic strategists, including some for other candidates, regard him more seriously today than they did at the outset.

Another question was whether former congressman Beto O’Rourke (Tex.) could transition his losing 2018 Senate bid into a serious presidential campaign. That was partly answered in the first 24 hours of his candidacy — when he announced he had raised $6.1 million, the biggest haul of any candidate’s first day and a sign that he would be among the beneficiaries of a new model of online fundraising.

At the same time, Democratic voters, in their zeal to find an alternative to President Trump, have shown a willingness to look beyond the traditional profile of a potential president — with Pete Buttigieg, the mayor of South Bend, Ind., surging from obscurity to become a factor and, last week, drawing more Google searches than anyone else in the field.

The field of 2020 presidential contenders are up against a unique set of challenges, from standing out on the campaign trail to dealing with President Trump. (Jenny Starrs/The Washington Post)

The leftward swing of Democratic voters has been reflected in the issues that have dominated in the past several months, including Medicare-for-all, the Green New Deal climate change proposal and reparations for descendants of slaves, among others. But more-moderate candidates have different views, foreshadowing a battle that will play out in coming months.

As those ideological tussles await, the collective field has continued to be overshadowed by a president who has been at the center of the country’s attention for nearly four years and probably will be until November 2020 — and possibly beyond.

The biggest single event of the quarter — and one that eased some pressure on Trump — was special counsel Robert S. Mueller III’s report, which did not establish a criminal conspiracy between Russia and Trump’s campaign during the 2016 campaign.

The Mueller report has not been made public yet, but already its reported findings — and Attorney General William P. Barr’s decision that a charge of presidential obstruction was not warranted — have changed the parameters for 2020: Democrats probably will have to try to defeat Trump the old-fashioned way — at the ballot box — rather than with an assist from prosecutors or an impeachment process.

The field now numbers 16, and several other people are deciding whether to join the race. The span of the candidates — by gender, race, age and ideology — has highlighted that Democrats are a party in transition, navigating a new landscape and a potentially narrow path through the electoral college.

“Honestly this is the most interesting, unfathomable and formidable field that I can remember,” said David Axelrod, the longtime Democratic strategist. “Really, really substantial candidates. Different profiles and approaches. And no clear indication yet which will prevail. It makes for a great campaign. And Democrats need the campaign to sort these things out.”

The field includes some of the most accomplished and also some of the least experienced politicians, whose ages cover four decades, from Sanders, 77, and Biden, 76, at one end to Buttigieg and Rep. Tulsi Gabbard (Hawaii), both 37, at the other. Trump is 72.

The size of the field, however, complicates the Democrats’ efforts to draw a clear contrast with the president. At this point, candidates are busy introducing themselves while Trump has a megaphone to attack them collectively.

Amy Dacey, a former chief executive of the Democratic National Committee, said she worries that no central message is emerging. “The Democratic Party has to show that we’re the alternative to the current power structure,” she said.

One pressing indicator will be the first-quarter fundraising reports that will begin to emerge before the April 15 deadline. They will not only stratify the field in ways the polls do not but also tell the story of the near-revolutionary way Democrats fund their candidacies.

What once was a world dominated by bundlers and big checks from wealthy individuals is now one in which tens of thousands of small contributions from citizens can continue to fuel campaigns. Sanders used that system to stay in the 2016 race until its end, and O’Rourke did the same during his Senate race.

“The ability to raise millions of dollars in small in increments has become critical,” said Guy Cecil, who chairs Priorities USA, a Democratic super PAC. “That is a huge change even from four years ago.”

The populist mood of Democratic voters also has forced a second notable change in fundraising: Super PACs, which had become a fixture of campaigns and provided a vehicle for rich donors to have a greater effect, have been almost nonexistent this time. But for those who cannot count on the grass-roots, small-dollar support, fundraising remains a time-consuming and laborious part of campaigning.

Sen. Cory Booker (N.J.) spent time in California’s Silicon Valley, while Harris has been locking down big donors and holding fundraisers in money hubs such as Atlanta, Dallas and Hollywood. Warren, whose attacks on big banks and corporations have limited her ability to raise large contributions, has tried to turn that problem into an asset, pledging not to hold any such fundraisers.

Biden has been fretting about whether he would be able to raise a large sum of money from the start. His aides have been contacting past supporters, seeking updated phone numbers and making sure donors would be ready. But he hasn’t been alone on a ballot since 2008 and, never a prolific fundraiser, he could be hamstrung by a new fundraising environment that rewards candidates who can amass quick sums online.

“What an inversion of how campaigns have been funded before,” said Dan Pfeiffer, an adviser to President Barack Obama and co-host of “Pod Save America.” “Biden is a former vice president who has been in politics for years — probably knows every bundler, every big donor that has been in politics for decades. And he’s worried about being outraised by a former three-term congressman because of online fundraising.”

Democratic strategist Jim Messina noted that with expensive states such as California and Texas now near the front of the campaign calendar, there is an even greater premium on the ability to raise sizable amounts of money. “The party that hates money — we just made it more important,” he said.

Perhaps because of their diverse challenges, the campaigns have employed vastly different strategies. Some are running a national race, while others have focused more on early voting states. Some are releasing policy after policy, while others are holding listening tours. Some mention Trump often while others treat him like Voldemort, he who shall not be named.

Warren has built up a political battleship of more than 100 staff members — including 65 placed in four early voting states and four advisers helping her pump out policy — albeit one that will require strong fundraising to sustain. Her campaign has kept track of almost everything, including how many people attend her events, the number of selfies she has taken (more than 12,000, at a rate of 450 per hour) and questions she has fielded from voters (199).

Her policy proposals have been aimed at what she calls deep structural changes in the economy and the political system. Yet Warren is also in the unusual position of having her standing decline from pre-campaign expectations, to the point that last week her campaign manager sent out a fundraising appeal spelling out why she is electable.

Harris has drawn among the campaign’s largest and most diverse crowds, building upon an ability to ignite a bulwark of the party’s base: African American women. She recently announced a plan to significantly increase teachers’ pay, a proposal pleasing to another cadre of Democratic voters. But questions remain about her more conservative record as a California prosecutor.

Sanders’s resilient corps of supporters — demonstrated at giant rallies in multiple states — has surprised others in the race.

“His strength feels much deeper and more concrete than I anticipated,” said a senior staff member in a rival campaign. “That’s the biggest thing that feels most telling in this quarter.”

Sanders, like Trump, is scrappy and combative, and his ardent, vocal following is intent on antagonizing his rivals. His huge grass-roots fundraising operation will keep him fueled indefinitely. That has spawned fears by some opponents that, like Trump in 2016, he could win primaries with a relatively small plurality in a crowded field.

O’Rourke has started at a frenetic pace, hopping on top of any countertop he could find and quickly cramming several months of campaigning into his first 10 days. During that period, he logged 2,366 miles behind the wheel of a rented Dodge Grand Caravan, holding 51 events across eight states and answering 357 questions from voters.

But fatigue with O’Rourke — and his ability to get Vanity Fair cover treatment — is palpable among rival campaigns. Aides have noted that Harris, too, has stood on chairs, and that Warren has spoken far more specifically about issues. An aide to Sen. Kirsten Gillibrand (N.Y.) gamely noted that she, like O’Rourke, has run a 5K race. When Gillibrand posted a video online of her lifting weights in a T-shirt — one with the phrase “Just trying to get some ranch,” an homage to a viral moment when an Iowan interrupted her appearance in search of salad dressing — it got 1.2 million views.

At this early stage, Democratic enthusiasm about the candidates is one of the most consistent themes and potential positives. One man flew from San Francisco to Waterloo, Iowa, to attend a party for O’Rourke. Two gay men, on their third date, drove eight hours from Tampa to Georgia to see Harris.

“The only conclusion anybody should draw right now is they shouldn’t draw any conclusions,” said Joel Benenson, a top strategist for Obama and Hillary Clinton, the past two Democratic nominees. “The last campaign we really only had two candidates all the way through. Last time we had a contested race was 2007 and ’08. And that started smaller and winnowed down quickly. I can’t think of a Democratic field comparable to this.”

Annie Linskey and Chelsea Janes contributed to this report.