HOUSTON — Candidates for the Democratic presidential nomination are sprinting from coast to coast in search of campaign donations over the next 18 days, moving urgently to stockpile cash for their big fall push — and to avoid a death spiral that a weak third-quarter fundraising tally might prompt.

With the third debate behind them — the fourth is in mid-October, well after the fundraising deadline — the candidates are devoting themselves to finishing with a flourish what is typically a difficult three-month fundraising period, according to a review of invitations and interviews with officials and donors.

Former vice president Joe Biden’s campaign has tentatively arranged at least 16 post-debate fundraisers before the Sept. 30 deadline, including one here Friday and another in Dallas on Saturday. Sen. Kamala D. Harris (Calif.) has at least 17 planned. Pete Buttigieg, the mayor of South Bend, Ind., was scheduled to hold at least 10, including one in Dallas on Friday.

Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (Mass.), who have opted not to host fundraisers catering to wealthy patrons, are revving up their online operations to rake in smaller contributions. The Sanders campaign says it is closing in on 1 million donors and hauled in more than 13,000 contributions on a single day last week. Many Democrats anticipate Warren will also post big numbers when the reports are released in October.

Still, Democratic donors have expressed nervousness in recent weeks that some presidential hopefuls could post disappointing totals, compounding the candidates’ broader struggles. July and August tend to be slow for fundraising, with many people on vacation and tuned out of politics. The large and unpredictably fluid field also has made it difficult for donors to commit to a candidate.

“The third-quarter number, from a finance standpoint, will define the narrative throughout the course of the fall, when these questions about viability for so many of the candidates are so real, especially in the second and third tiers,” said Rufus Gifford, the finance director for President Barack Obama’s 2012 reelection campaign and a donor to at least three candidates so far this year.

Steve Westly, a California investor raising money for Biden, predicted Friday that Biden’s debate performance, which received positive reviews, would help his fundraising. “Every time he puts in a good performance like this, I think he just closes the door a little bit more on the other candidates,” Westly said.

However, the full effect of the third debate probably will not become clear for weeks. In the weeks beforehand, some donors questioned whether Biden, the leader in the polls, would be able to stay at the front of the money pack after a tough stretch marred by gaffes and misstatements. They worried, too, about his ability to sustain momentum among small-dollar donors.

Financial question marks also surrounded other candidates. Some Harris donors expressed concerns about her shifting rhetoric on health care. There were doubts about whether Buttigieg, who led the pack with a huge haul in the second quarter of the year, could keep up his strong pace after stagnating in the polls. Meanwhile, Warren and Sanders have been battling for supremacy on the small-dollar field.

By Oct. 15, each candidate will have to submit a report to the Federal Election Commission detailing what they raised and spent between July and September. Those figures will be available to the public, offering a snapshot of what kind of financial shape the campaigns are in as they push toward voting in February.

Third-quarter hauls can signal to supporters and donors whether campaigns can survive the early primary contests, the first two of which could cost upward of $75 million per candidate. And they can provide a measurement of candidate enthusiasm.

Many of the party’s biggest donors and fundraisers are giving to multiple candidates to play the field or not giving at all. The campaigns’ third-quarter numbers will help bring clarity to donors who are sitting on the sidelines or who are still deciding which candidates to take off their list, fundraisers say.

“Third-quarter fundraising matters to the people at the bottom end of the totem pole. They can’t pay for buses in Iowa. There’s a practical reality here,” said Tom Nides, a longtime Democratic fundraiser who has given to at least two presidential candidates this year. “If you’re only raising $3 million to $4 million per quarter, and it takes $6 million, it doesn’t take a rocket scientist to know that’s not sustainable.”

Some candidates have successfully used past debate performances to gain fundraising momentum. For example, after a headline-grabbing performance in the first debate in June, Harris saw a donation bump that propelled her to a top-five haul in the second quarter.

But she struggled to keep her momentum, slipping in the polls in subsequent weeks. She faced questions about her statements on school busing and her position on abolishing private health insurance. At a meeting after the first debate, some of her fundraisers expressed concern that her early slip-ups could become a lingering problem.

“They said, ‘Listen, the health-care equivocation or mistakes are important. We’re not going to let that slide,’ ” said one Democrat who was present and spoke on the condition of anonymity to describe private conversations with other fundraisers. “The same way we tried to tell Hillary Clinton’s people in August 2015 that the emails are a big deal, and you can’t just poo-poo the emails.”

Harris, who raised about $12 million in the second quarter, held a fundraiser here on Friday morning and was slated to tap some celebrity star power later in the day at a Chicago fundraiser featuring Star Jones, a former co-host of ABC’s “The View.” On Sept. 23, Harris will hold a California fundraiser hosted by actor Don Cheadle.

Biden, who has held his lead in most public polls, has already held 23 fundraisers this quarter. He has an aggressive fundraising schedule for the rest of September, with events scheduled for him or his wife, Jill Biden, in Florida, New York, California and other states.

Biden raised about $22 million between his entry into the race on April 25 and the end of June. His total was the second largest in the Democratic field — eclipsed only by Buttigieg, who brought in nearly $25 million, albeit over a longer period.

The former vice president has continued to hold fundraisers with wealthy donors, which has drawn criticism from some elements of the party worried about the influence of the rich and powerful in the primary contest.

Facing blowback from environmental activists, Biden defended his decision to participate this month in a Manhattan fundraiser co-hosted by the co-founder of a natural gas company. “I just want to be very clear to everyone here: I am committed to not raising money from fossil fuel executives, and I am not doing that tonight,” he said.

Biden said the event was not a violation of the pledge because the co-founder was not a company executive, as defined by U.S. Securities and Exchange Commission filings.

In the face of criticism, Biden’s campaign has touted its fundraising transparency. It has invited a reporter to every fundraiser that the former vice president attends to file a dispatch distributed broadly to the news media.

Buttigieg is among the candidates who held multiple fundraising events through the summer in the elite vacation havens of Nantucket, Martha’s Vineyard and the Hamptons. But his schedule has slowed down amid the late-summer fundraising lull, his campaign said.

“I think we’re going to continue to be able to be extremely competitive, and most importantly, to gather the resources we need for the ground game that’s actually going to help us win,” Buttigieg told reporters this month.

Warren and Sanders had big second quarters, bringing in about $19 million and $18 million, respectively. The totals, though impressive, set high expectations for the rest of the year.

Sanders campaign officials voiced optimism about their fundraising pace, which relies largely on an extensive email list and a loyal network of supporters who frequently donate in small increments. Sanders also has held a handful of what his campaign calls “grass-roots fundraisers” with modest entrance fees.

“The advantage of relying on grass-roots fundraisers is we can be in a gym in Iowa eating pizza with real people and talking about the issues that matter to them, as opposed to sipping Prosecco and eating canapés in a penthouse,” said Ari Rabin-Havt, deputy campaign manager for Sanders.

Warren, who has promised not to hold in-person fundraisers with wealthy donors or conduct “call time” to solicit and thank donors giving maxed-out checks, has impressed some traditional Democratic fundraisers and donors with her ability to build a grass-roots fundraising machine in a relatively short time.

Now, they are watching for her figures with great interest. Although Warren has shunned the “bundler” class of fundraisers who collect big checks on behalf of the candidate, some fundraisers said they are getting calls from donors asking whether Warren is looking for help raising money.

“We all want to help her in case she becomes the nominee. I want to have a link” to her campaign, said the Democratic fundraiser who was at the Harris meeting and is raising money for multiple candidates.

The fundraiser added: “People like me were ignoring her for June, July and August, but in the last week or so, we’re like, ‘Hey man, Elizabeth is up there in the top three.’ If Elizabeth Warren is the nominee, we’re all in. We’re all raising her money.”

Lee reported from Washington.