Last month in Manhattan, Beto O’Rourke held a private reception for supporters who had paid the maximum amount to his campaign or brought in as much as $25,000 by persuading others to do the same.

It was the first such fundraiser of O’Rourke’s presidential bid — and a contrast from the early days of his campaign, when he emphasized that he had “no large-dollar fundraisers planned, and I don’t plan to do them.”

Across the Democratic field, candidates are embracing the big donors they distanced themselves from early on — a sign of increasing doubt that the small, online donations the campaigns have been chasing will be sufficient to sustain two-dozen primary contenders.

Many of the candidates previously had held a handful of high-dollar fundraisers or avoided them altogether, seeking to tap into the populist sentiment that has animated the Democratic base. They tried to capi­tal­ize on the deluge of online donations that helped fuel the midterm elections, rather than making the traditional overtures to wealthy donors, once a staple of early presidential campaigns.

But after a disappointing fundraising haul in the first quarter of the year, and as the primary drags on with no clear front-runner, many of the candidates are turning their focus to wealthy donors — a strategy that could help keep their campaigns viable but may hamper their ability to connect with base voters.

“There’s not an unlimited pool of cash, even in the small-dollar sense,” said Tom Nides, former deputy secretary of state for management and resources under President Barack Obama and a longtime Democratic fundraiser. “The candidates have no choice now. They’re going to have to do both, like it or not. They’re all doing it.”

The eventual Democratic nominee will challenge President Trump’s giant fundraising machine, which has already raised more than $168 million on the strength of both small-dollar and wealthy backers.

The stakes are getting higher, with just one month to go until the next deadline for campaigns to report fundraising figures. The numbers made public after June 30 are expected to help sift potential front-runners from the rest.

“Everyone got a bit of a free ride in the first quarter because expectations were low across the board,” said Ami Copeland, former deputy national finance director for Obama’s 2008 campaign. “If there’s any dip in what they raised before, that will be a telltale sign of them having some problems.”

This weekend, several candidates were scheduled to appear at fundraisers in the San Francisco Bay area, including Pete Buttigieg, Sen. Kirsten Gillibrand (N.Y.) and Sen. Amy Klobuchar (Minn.).

Sen. Bernie Sanders (I-Vt.), who famously built his campaign around small donations in 2016, on Saturday held his first fundraiser of the primary this weekend in San Francisco. However, he has priced the lowest-tiered ticket at $27 — the average donation to his campaign in 2016.

Sen. Cory Booker (N.J.), Gillibrand and Klobuchar recently schmoozed with the “Hillblazers,” who raised at least $100,000 for Hillary Clinton’s 2016 presidential campaign, at a gathering at the D.C. home of Esther Coopersmith, a major Clinton ally, according to an attendee.

And on Wall Street, a group of Democratic donors recently met with Buttigieg, the mayor of South Bend, Ind. One person who was present and spoke on the condition of anonymity to describe the private meeting said: “It felt like bringing Marilyn Monroe into the room and all these guys saying, ‘Who’s she? Where’s she from? And when is she coming back?’ ”

Some of these efforts are already bearing fruit.

In Los Angeles, some of the wealthiest Democratic donors have made maximum $2,800 contributions to more than a dozen Democratic presidential candidates, according to someone familiar with the contributions. The person spoke on the condition of anonymity to describe private matters.

In New York, a group of wealthy and prominent Democratic donors recently opened their homes and offices in Manhattan to host O’Rourke, Buttigieg, Klobuchar, Booker and Sen. Michael F. Bennet (Colo.) for private “salons,” according to a person familiar with the gatherings, who spoke on the condition of anonymity to discuss private events.

Many of those donors have made maximum donations to multiple candidates, the person said.

One of those donors is Robert Wolf, a former investment banker and an Obama supporter. He said he has met with about a dozen candidates, which included a recent one-on-one with O’Rourke, and has given money to 10 presidential contenders.

“We are more interested in beating President Trump than we are in getting the candidate we like and/or know well,” he said.

The big checks from such donors are becoming crucial to surviving the primaries. With such a large field, the limitations of raising tens of millions of dollars from donations of under $200 at a time are beginning to show. Fundraising figures released after the first three months of 2019 showed that the 16 Democratic candidates in the race at the time had together raised nearly $90 million — slightly more than what eight Democrats raised at the same point in the 2008 cycle.

The field has ballooned since then to 24 and shows no sign of winnowing.

“People are really trying to find out: What is the ideal recipe to be successful?” said Taryn Rosenkranz, founder of the political-fundraising consulting firm New Blue Interactive. “Every candidate is like, ‘Wow, there’s just so much more I have to do, because it’s that much more competitive.’ ”

Campaigns are also finding that, despite what felt like an endless spigot of small-dollar donors to tap, they are, in fact, vying for the same pool of supporters: loyal Democratic primary voters.

The digital firm Bully Pulpit Interactive found that campaigns are competing for the same pool of donors on Facebook. It found that 43 percent of the money spent by campaigns is targeting base voters in 14 Democratic-stronghold states, rather than trying to reach broader audiences.

Despite the fundraisers he has planned, O’Rourke remains committed to grass-roots donors — a strength he showed in his Senate candidacy, according to his campaign.

“Beto held zero fundraising events during the first two months of his campaign as he visited 14 states and held 150 town halls, where he took over 1,000 questions from those he wants to serve,” said Chris Evans, an O’Rourke campaign spokesman.

O’Rourke is one of the few Democratic candidates to publicly disclose portions of his private fundraisers. His remarks at the New York event last monthaired on Facebook Live, though the question-and-answer portion and the pre-function receptions were not shared publicly. The video has been viewed more than 42,000 times, including by those who donated to his campaign online while watching in real time, according to the Facebook comments.

Like most Democratic candidates, O’Rourke pledged early in his campaign not to take money from certain special-interest groups and political action committees. He had not ruled out holding fundraisers.

Now that O’Rourke has begun holding such events, he will be able to tap into the network of donors who coalesced around his 2018 Senate campaign, which shattered fundraising records.

The only candidate who has completely shunned in-person fundraisers during the presidential primaries is Sen. Elizabeth Warren of Massachusetts.

Recognizing that Warren is at a disadvantage, smaller groups of fundraisers are forming in various parts of the country to help collect larger amounts of money. These volunteer fundraisers said they have notified the campaign of their efforts to hold events, make calls and send emails on her behalf.

For example, in Los Angeles, a group of about 50 television writers are coordinating their efforts through Slack, an online messaging community, to raise money for Warren. They are planning to launch “Warren Wednesdays” to hold weekly fundraisers and are hoping she might bend her rule against in-person fundraisers by appearing via Skype.

Eric Spiegelman, who is leading the effort, said Warren’s no-fundraisers pledge is a challenge her supporters can take on to raise money on her behalf.

“She’s made it clear that she doesn’t want to show up at the big-dollar fundraisers. I know exactly the types of events she’s talking about,” Spiegelman said. “It makes me feel as though she really needs our help.”

Michael Scherer contributed to this report.