(CNN) Lower-polling Democratic presidential contenders are coming to grips with the reality of the 2020 race : More quickly than they'd expected, they are running out of time -- and opportunities -- to change their trajectory.

Increasingly, candidates are being crowded off the debate stage and out of the national news cycles. The House's move to begin impeachment proceedings against President Donald Trump means there is less attention on the 2020 Democratic primary than many campaigns had hoped for. And the Democratic National Committee's rising thresholds to qualify for the debate stage in November are so far proving difficult for more than seven or eight candidates to achieve -- leaving those who don't make the stage with little hope of breaking out in front of a large national audience.

As the fall begins, with just four months left before the Iowa caucuses, some candidates and campaigns are taking the rare step of saying publicly that they are already facing make-or-break moments.

New Jersey Sen. Cory Booker was first to try an attention-grabbing gambit: His campaign said he needed to raise an additional $1.7 million in the final 10 days of the third fundraising quarter, which ended Monday night -- and if he fell short, he would drop out. Booker hit that mark Sunday, with a little more than 24 hours to spare.

Former Housing and Urban Development Secretary Julián Castro set a different bar in a fundraising email last week, in which he told supporters he needed to be "completely vulnerable" with them: If he doesn't meet the DNC's higher polling and fundraising thresholds for the November debate, "it will be the end of my campaign."

"The DNC's new debate threshold will be the end of the road if we can't get more money in the door -- immediately," Castro said in the email. It's not just the lift from a minimum of 130,000 donors to 165,000 donors, he said -- it's the "millions" it will cost to run enough ads to hit the minimum of 3% in four polls.

Other campaigns are facing similar crunches. Several candidates who missed the cut in September, when the debates shrank from 20 contenders over two nights to 10 over one night, have either dropped out already -- see New York Sen. Kirsten Gillibrand and New York City Mayor Bill de Blasio -- or their aides realize that time is running out.

Challenges ahead

The Democratic field has been relatively constant in recent months, with a clear top five

Massachusetts Sen. Elizabeth Warren and former Vice President Joe Biden lead most polls of likely Democratic primary voters nationally and in the four early-voting states, with Vermont Sen. Bernie Sanders typically in third place. South Bend, Indiana, Mayor Pete Buttigieg and California Sen. Kamala Harris round out the top tier, and while both have lost altitude since the early summer, they remain solidly positioned to continue qualifying for debates.

After those five, things get murky.

The next tier includes five candidates who made the cut in the September debate: Booker, Castro, former Texas Rep. Beto O'Rourke, Minnesota Sen. Amy Klobuchar and entrepreneur Andrew Yang. All are polling in the low- to mid-single digits nationally and in the early-voting states.

Part of the challenge for lower-polling candidates who haven't kept pace with the top tier in fundraising, aides to several campaigns said, has been money. In this grouping, only O'Rourke -- who set a record when he raised $80 million as a Texas Senate candidate last year -- started with a large national email list. Other candidates have had to spend money on digital ads just to build their lists and attract the minimum number of donors the DNC has required. And, as O'Rourke's fundraising slowdown showed, even having those lists isn't a guarantee that the small-dollar donations will flow in such a large field with many voters and donors still considering multiple candidates.

Candidates have also had to gear their candidacies to appeal to a national audience, rather than focusing solely on voters in Iowa, New Hampshire, Nevada and South Carolina.

Meanwhile, influential national Democrats who could tip support in those lower-polling candidates' way are staying neutral, rather than offering endorsements.

For Booker, the decision to set a higher goal than other campaigns have publicized in the closing days paid off: He created a sense of urgency where there hadn't been one before among people who like his voice want him to remain in the race, even if they haven't yet decided who they will support.

"In order for our voice to stay in it, we had to sort of pull back the curtain and say, this is just the situation," a Booker aide said.

An aide said that Castro didn't sign off on the language in the email and campaigns often use bellicose language to drum up support near the end of a fundraising quarter, but the dire warning has been in keeping with Castro's campaign. Castro has said for months that fundraising and the ability to make the stage will be central to his success and he has backed that up by delivering a number of moments during debates.

"Secretary Castro has said from the start of this campaign that making the debate stage is critical to his success," said Sawyer Hackett, a Castro spokesman. "We are confident he will make it, and are counting on the support of grassroots donors to get us there."

The vulnerability Booker and Castro have shown is rare: Campaigns typically attempt to project strength at all times.

But in recent days, those that aren't as bluntly admitting the tough road they face are opening up.

Klobuchar has pledged to stay in the race through the Iowa caucuses, saying in a fundraising video last week that she is in it for the "long haul."

But in the video, she told supporters and donors that she is going to be "straight" with them.

"Someone suggested maybe I should tie myself to a railroad track and then record this video and say 'I won't move until I get all your donations,' in to a million dollars," she said in the video. "But I'm not going to do that because I give it to you straight."

Klobuchar's approach depends heavily on a stronger-than-expected showing in Iowa vaulting her forward, where a mid-September CNN/Des Moines Register/Mediacom poll showed her at 3% among likely Democratic caucus-goers. Klobuchar campaign manager Justin Buoen said the campaign spent the third quarter growing its organization in the early states -- and that Klobuchar has already exceeded the minimum number of donors to make the November debate.

In El Paso, Texas, O'Rourke's campaign has hoped that a new focus on gun control -- with O'Rourke advocating a mandatory buy-back of all assault-style rifles -- will give him a new audience.

His aides have also played up the reality that O'Rourke said he supported Trump's impeachment as far back as 2017, as a Senate candidate.

Yang, meanwhile, is moving ahead of several candidates in this tier, largely thanks to an online army dubbed the "Yang Gang." On Wednesday, he announced a $10 million haul for the third quarter, dwarfing his previous fundraising efforts.

Two candidates who have hopes for a fourth-quarter boost are billionaire investor and Democratic activist Tom Steyer and Hawaii Rep. Tulsi Gabbard. Both missed the qualification threshold for the September debate, but will be on stage in the October round in Ohio.

After them are several more candidates who look locked out of the debates -- at least in the near-term.

Tom Perez, the DNC chairman, has had little sympathy for the lower-polling performers.

"If you look at the history over the last 40 years or so," he told CNN's Chris Cuomo in mid-September, "nobody under 2% in the fall has won even a primary or a caucus."

Former Rep. John Delaney's campaign has been in turmoil for weeks. John Davis, Delaney's one-time campaign manager, and Will McDonald, his spokesman, cut ties with the campaign recently. Delaney's Iowa State Director Monica Biddix and New Hampshire State Director Chris Mackenzie also left the campaign last month.

Delaney, in an interview with CNN on Wednesday, said the departures stemmed from disagreements over the future of the campaign. Despite all but giving up on making a future debate stage and showing no movement in national or state-based polls, Delaney told CNN that he will be in the 2020 race until Iowa and plans to spend his own personal fortune to make that happen.

"Let's put it this way, the money I have put in this campaign I am never going to spend" personally, he said when asked whether his personal fortune is being wasted on the run.

Ohio Rep. Tim Ryan, like Gabbard, is fundraising simultaneously for his presidential bid and his congressional campaign -- a move that positions him to keep his seat if he is not the party's presidential nominee.

Montana Gov. Steve Bullock, meanwhile, is downplaying the importance of debates, even though he slammed the DNC thresholds after he failed to qualify for the first debate.

"There was polling that said, only one third of the folks even surveyed watched the debates," he said. "It's not like there's anything coming out of these debates that are really changing people's views."

He has pledged to make it to Iowa and run a shoestring campaign if that is what it will take. Bullock's team believes that the race remains fluid and people's minds are far from made up, giving a candidate like Bullock a chance to catch fire. But his close aides also admit that, if that is going to happen, it has to happen soon.

Colorado Sen. Michael Bennet, like Bullock, believes that the debates largely "aren't critical," an adviser told CNN.

"There was a decision made early on that the debates were not going to be his path," the adviser said. "And I think the way it has proven out is the debates haven't made it for anyone. Maybe you get a little bounce afterward ... but it doesn't seem to be sticking."

The adviser said Bennet, despite low poll numbers, plans to stay in the race through Iowa and New Hampshire, a goal that is possible because the Colorado Democrat has been thrifty over the last few months.