Democratic presidential candidates greet the audience on the second night of the debate hosted by NBC News, in Miami on June 27. (Byrnn Anderson/AP)

The prime-time Democratic debate in Detroit late this month could be a season finale for as many as half the candidates who have made the stage, as they struggle to meet the higher qualification requirements that will go into effect for later televised gatherings.

Just six candidates have met the combined polling and donor thresholds needed to appear onstage for the debate in mid-September, according to public surveys and self-reported donor counts. Four others are close to passing both tests, according to a Washington Post analysis of the field.

But for the remaining 14 candidates, including 10 who will appear on the July debate stage, securing a lectern and therefore a chance to speak directly to millions of voters gets much more complicated.

Top aides to several campaigns said they are either changing their strategy or effectively abandoning hope of being onstage in September, and aiming instead for the October debate, which has the same donor and polling thresholds.

Former Maryland congressman John Delaney’s campaign spokesman said the heightened requirements for geographically disparate donors that will be in place for September are forcing him to spend precious resources on ginning up national interest in a campaign that intended to focus largely on Iowa and other early-voting states.

The campaign has added trips to New York for television appearances, trying to conform to circumstances that are pushing candidates to appeal more to national concerns and emphasize sharply polarizing messages to gain more attention from potential backers, he said.

“You have to run these digital ads that are super-partisan and super-inflammatory to get your numbers” of donors, campaign spokesman Michael Hopkins said, describing what he sees campaigns doing. The major beneficiaries of the Democratic National Committee’s rules, he said, are Facebook and Twitter, the platforms where candidates place digital ads begging for money.

“We need to be talking about health care,” said Hopkins. “But when you talk about Trump and Russia, which are popular in the cable news cycles, you get on TV. What they are doing is forcing us to be cable news commentators.”

Other campaigns said privately that they, too, had spent more money on digital ads to try to reach the debate thresholds. Some have found that the cost of acquiring a single new $1 donor can exceed $50.

Several campaigns acknowledged the likelihood of being dismissed by party activists and voters if their candidates aren’t onstage in future debates. They worry that such an absence will prompt a death spiral, making it more difficult to attract the new donors and higher polling numbers needed to get onstage the next time. They also noted that activist groups holding cattle calls for candidates often decline to invite those lagging in money and polling strength.

“You’re sort of written off by the Acela corridor class if you don’t make it,” said Sawyer Hackett, a spokesman for former housing secretary Julián Castro, who has met the donor threshold for September’s debate but still needs higher poll numbers to qualify. “So yeah, that’s pretty important.”

The DNC plans to hold at least 12 presidential debates; the series began in Miami on two nights at the end of June, with 10 candidates onstage at each one. The debate on July 30 and 31 will be held under the same rules; the candidates who qualified for that debate were apportioned by sponsor CNN to two stages on Thursday night.

Candidates could participate in the June and July debates if they reached a target of 1 percent in three polls, or received donations from at least 65,000 people, including at least 200 in 20 states.

But to get onstage for the September and October debates, candidates will be required to meet both thresholds, which doubled to 2 percent in four polls and a minimum of 130,000 donors, with at least 400 donors in 20states.

“You have to show progress,” said DNC spokeswoman Xochitl Hinojosa.

The DNC and media partners ABC News and Univision have set aside two evenings for the September debates, so there will be slots available for up to 20 candidates. But that second night might not be needed.

Of the 20 candidates who qualified for the July debate, just 14 of them met both the donor test and the polling test that were in place for the summer debates. Six others made the stage since the DNC allowed candidates to qualify via either the donor requirement or the polling test.

Several candidates have disclosed receiving just a fraction of the required number of donors — 6,700 in the case of New York Mayor Bill de Blasio and about 14,000 in the case of former Colorado governor John Hickenlooper — as of the end of the second quarter. Several other campaigns have kept their donor totals secret, suggesting that they have not yet hit the threshold.

Ten of the candidates who will appear onstage in July — including Sens. Kirsten Gillibrand (N.Y.) and Michael F. Bennet (Colo.), and Rep. Tulsi Gabbard (Hawaii) — have not received 2 percent in any party-sanctioned polls since the June debate. To qualify for the September gathering, they and the others will need to achieve that hurdle in polls released between June 28 and Aug. 28.

“The 2 percent is going to be the killer,” said an adviser to a campaign that has made the first two debates. “Two percent sounds really small until you talk about a 25-person field.”

Another hurdle for campaigns trying to gain ground in polling: Pollsters typically cut back on surveys during vacation periods, when many voters are thinking more about recreation than national politics.

“It’s a holiday season where people aren’t really focused,” said Hopkins. “In every way that the DNC could have messed this up, they’ve messed this up.”

Hinojosa, the DNC spokeswoman, noted that the party has always increased the thresholds to qualify for debates as campaign seasons wear on. She said that adding a donor requirement is designed to ensure that the eventual nominee will have a strong grass-roots funding base, which the party believes will be needed to take on President Trump.

“When we did our briefing with all of the campaigns earlier this year, we were very clear what the thresholds would be and that thresholds would go up for future debates,” Hinojosa said. “And no one objected.”

The DNC’s goal is to make sure that only candidates considered viable are on the debate stage. Yet several campaigns insist that their candidates will stay in the race even if they aren’t allowed onstage.

“I don’t think that these debates are going to be dispositive,” said Montana Gov. Steve Bullock, a recent presidential entrant who missed the cut for the June debate but will be onstage in July. He has focused his campaign on Iowa.

“I mean, I think it still is going to be the early states that winnow down the field . . . That’s what I thought about and banked on,” Bullock said.

Bullock acknowledged it will be hard for him to pick up enough donors to make it to September’s debate. He said he will “take it as it comes.”

Bennet’s operation also sees hope of breaking out of the pack at a later date and has made clear that he is prepared to continue in the race even if he loses access to the early fall debates.

“The polling these last few weeks if anything has shown that this race is less consolidated than it was six months ago,” said Shannon Beckham, a Bennet adviser.

A similar case is made by strategists for Rep. Seth Moulton (Mass.), who point to polls showing that most Democrats have not made up their minds about a candidate. Moulton did not qualify for the June or July debates.

“Not being on a debate stage is not an incentive to get out of a race that is still wide open,” said Matt Corridoni, an adviser to Moulton. “The debate is one day on a calendar, and it should not make or break your campaign.”

Moulton’s campaign is focused on New Hampshire, South Carolina and Nevada, where there are relatively higher shares of his fellow military veterans.

Still, Moulton’s campaign sent a letter to the DNC on Wednesday that took issue with the criteria it used to determine which candidates can participate.

“Seth Moulton, a Marine Corps veteran who served four combat tours in Iraq, is running in the Democratic presidential primary on a platform of keeping our country safe and secure, taking back patriotism for our party, and encouraging every American to serve our nation,” wrote Marie Harf, a top aide to Moulton’s campaign. “These issues need to be discussed in our party’s debates if we want to select the best candidate to take on and beat Donald Trump.”

After the first debate, Hickenlooper experienced a massive campaign shake-up prompted by the DNC’s release of the September debate qualifications.

Several top advisers urged Hickenlooper to consider ending his campaign after realizing the likelihood that he would not make the September bar because of difficulties attracting donors. His press adviser, campaign manager and top fundraiser all left the campaign after the Miami debate.

“We are going to keep campaigning,” said Hickenlooper’s new spokesman, Peter Cunningham. “We are going to spend a lot of time in Iowa. The hope is that he gets a bump in the polls there, which will get some national attention and bring in some new people.”

That sentiment was echoed across the sprawling field.

“The goal is to fight through the rest of August” to make the stage, said Olivia Lapeyrolerie, a spokeswoman for de Blasio, who will be at July’s debate but is far from either the polling or the donor requirement for September.

John Wagner contributed to this report.