WASHINGTON (CNN) Colorado Sen. Michael Bennet, who joined the crowded presidential field on Thursday, may be a longshot candidate for the White House.

But as the 21st major candidate to seek the Democratic nomination, his entry into the 2020 race underscores the increasingly high stakes for Democrats looking to make the early debate stage. And some campaigns have begun to openly bristle at the national party's use of a 65,000-donor threshold as a requirement to join the first two debates.

The Democratic National Committee's nationally televised presidential debates in June and July will include as many as 20 candidates who either register at least 1% in three national or early nominating-state polls or who attract 65,000 unique donors. The donor pool must include at least 200 people in each of 20 states.

But if more than 20 candidates qualify, the DNC will use both the polling and the donor threshold to winnow the field.

As of Thursday morning, several well-known politicians who have met the polling threshold had not notched 65,000 donors. The list includes Washington state Gov. Jay Inslee, New Jersey Sen. Cory Booker and former Housing and Urban Development Secretary Julian Castro.

With Bennet now in the race and other potential candidates still considering bids, the scramble to lock down more donors was evident.

Stressful for donors?

Julian Castro, a former Cabinet secretary in the Obama administration and the only Latino candidate in the field, needs more than 2,000 additional donors to meet the threshold, he said in a new fundraising video his campaign released Thursday.

His spokeswoman Jennifer Fiore said she expected Castro would meet the donor criteria by week's end. But she raised questions about the requirement, saying it has proven "stressful" for the party's small donors.

"They are saying, 'Hold on, are you telling me that my candidate might not appear on the debate stage when he's the only person talking about my issues?'"

"At some point, that's going to take a toll," she added.

Privately, some campaigns are arguing that the approach is enriching consultants and ad vendors while forcing smaller campaigns to waste money trying to inflate their donor numbers.

"The DNC is incentivizing campaigns to do non-strategic things," said a senior adviser to one of the presidential campaigns, "and if they raise the bar for subsequent debates, it'll just continue."

The DNC's donor threshold also is "incentivizing bad email fundraising," said Tim Lim, a partner at NEWCO Strategies, a consulting firm that works with campaigns, organizations and brands. "They're trying to turn people into donors way too quickly."

Meanwhile, digital ad spending has exploded in the 2020 race as campaigns attempt to grow their small donor numbers.

"My concern is, it's not an accurate barometer of whether you have actual grassroots support," Lim said. "It's a math problem."

And he was skeptical that the DNC would enforce its thresholds if it were to mean excluding qualified candidates who have served in Congress, as governors or in the Cabinet -- the sort of candidates who, in past presidential primaries, would have made the debate stage without question.

"I ultimately think the DNC will have to change their rules, since it looks like over 20 candidates will qualify for the debate," he said.

Adrienne Watson, a DNC spokeswoman, said the party will not retreat.

"The DNC, along with its network partners, released the threshold for the first two debates nearly four months prior to the debates," she said in an email to CNN. "It was set before all the candidates were in the race and it remains as candidates enter the race. It will not be revised now."

Fundraising appeals

Within an hour of Bennet's announcement Thursday, Booker issued a fundraising plea, telling supporters that he was just 1,971 contributors away from meeting the 65,000-donor threshold needed to "guarantee" a place on the debate stage. By 1 p.m. EDT, another fundraising appeal from Booker's campaign put the number of donors still needed at 1,592.

"With the largest presidential primary field we have ever seen, we want to be in the most competitive position possible," Booker campaign spokesperson Sabrina Singh said. "If there ever needed to be a tiebreaker to make the debate stage, we wanted to ensure that we met both polling and the donor threshold."

Other candidates also are spending money to bring donors into the fold.

In his Facebook ads this week, Inslee -- who has made climate change the center of his presidential campaign -- asks his supporters to become a "debate donor right now."

Inslee has added "tens of thousands of donors in the last weeks," his spokesman Jared Leopold said. He said Inslee is "building momentum towards 65,000 donors."

Former congressman and self-funder John Delaney had landed on his own strategy to attract to requisite number of donors: He's offering to give to charities on donors' behalf if they chip into his campaign.

Delaney spokesman Ahmed Elsayed declined to say how many donors the former congressman has attracted so far. He said campaign officials are "confident" Delaney will make the debate stage.

DNC officials plan to certify who has qualified for the debate 14 days before the first event, slated for June 26 and June 27 in Miami. NBC, MSNBC and Telemundo with partner with the DNC on the June debate.

The fundraising pressure will only grow in the months ahead.

The party plans to use higher fundraising and polling thresholds for candidates to qualify for debates later in the primary campaign.