Westerville, Ohio (CNN) For four Democrats, the CNN/New York Times presidential debate on Tuesday night could have been their last.

Sen. Amy Klobuchar of Minnesota, former Rep. Beto O'Rourke of Texas, former Housing and Urban Development Secretary Julián Castro and Rep. Tulsi Gabbard of Hawaii walked onstage here at Otterbein University with a stark reality looming: This could be their last debate, as none of them have yet qualified for the November contest.

To make the November debate stage, candidates will have to reach 3% in at least four Democratic National Committee-approved polls of Democratic voters nationally or in one of the four early-voting states -- Iowa, New Hampshire, Nevada and South Carolina -- and raise money from 165,000 separate donors -- including a minimum of 600 donors each in at least 20 states or territories.

The reality for the four Democrats on the bubble is that none of them are that close to making the fourth debate. All the candidates have the needed donors, but only O'Rourke and Klobuchar have one of the needed polls.

While the four candidates enter the coming weeks of campaigning with a shared goal of appearing on the November stage, the distinctly different Democrats used different methods Tuesday night to set themselves apart.

Where Klobuchar was aggressive and direct, Castro -- who had been forceful in earlier contests -- was more muted and attacked less. Where Gabbard looked to score points by hitting her opponents at the end of nearly all of her answers, O'Rourke tried to be more selective in when to strike at his opponents.

The varying approaches highlight not only different political stances among the Democrats, but also divergent strategies on how each believed they will be able to drum up enough support to make the next debate.

Time for a 'reality check'

Klobuchar is the candidate who made the most notable change from her past appearances. In the first three debates, she was more muted, operating like she was alone on the stage and there only to make her particular policy points. On Tuesday, however, Klobuchar -- from the outset -- used her time to press Sen. Elizabeth Warren of Massachusetts, taking on the liberal lawmaker as a way to touting her own moderate bona fides.

Klobuchar accused Warren, now a front-runner in the primary contest, of being dishonest.

"We owe it to the American people to tell them where we will send the invoice," she said of Warren's continued dodge on whether middle-class taxes will go up to pay for the single-payer "Medicare for All" proposal she backs.

When Warren suggested that people onstage who disagree with her want to protect billionaires, Klobuchar shot back.

"I want to give a reality check to Elizabeth because no one on this stage wants to protect billionaires," she said. "Not even the billionaire wants to protect billionaires," she added, referring to businessman Tom Steyer.

Then she looked to undercut Warren's central argument: that she is in the race to fight for regular people, a suggestion that others are not.

"I think simply because you have different ideas doesn't mean you're fighting for regular people," Klobuchar replied. "I wouldn't even be up on this stage if it wasn't for unions and the dignity of work."

The performance was at a speed that few watching the earlier debates had seen from Klobuchar.

"Amy made the clearest case tonight," said Christina Reynolds, a top staffer at Emily's List and former aide to Hillary Clinton. "She looked at it and said, 'I'm going to make the clear case for being the moderate candidate.' .... That's her case. And she made that case really well."

While Klobuchar denied that she had been any different on Tuesday night -- "I have been literally the same person from the beginning. This debate was longer and I got more airtime and I had more of a chance to do it," she said -- a senior aide to the senator said the prospect of missing the November debate was "in the back of her mind some."

"She's been saying the same thing, but we set it up more aggressively, more forcefully today, because I think it's more important than ever for people to understand," the aide said.

For his part, Castro went away from the more pointed strategy he had employed in September's debate -- one that earned some negative reviews -- and delivered a more subdued approach.

Castro, in the most-buzzed-about moment of his night, accused Trump of "caging kids on the border" and "effectively letting ISIS prisoners run free" in Syria. But his lack of aggression toward his fellow Democrats stood out, especially after the Texas Democrat had repeatedly gone after Joe Biden in the Houston debate over the former vice president's memory.

Castro insisted there had been no change in strategy on Tuesday.

"What I felt is that I needed to go out there and to continue to articulate a strong, compelling vision for the future of our country," he said. "And I think that I did that."

But the reviews from unaffiliated Democrats were not glowing.

"Castro totally faded," said Mo Elleithee, a longtime Democratic operative. "When you are on the bubble, you cannot afford to fade into the background. You have to find the opportunities."

Like Castro, O'Rourke denied that the prospect of making the fifth debate was on his mind as he took the stage on Tuesday.

"I really wasn't" thinking about it, he said. "I'm really focused on making sure that I see things as clearly as possible, speaking about them as honestly as possible."

And O'Rourke's strategy backs that up: The former congressman delivered an even debate performance that at times mimicked the directness he has brought to past contests.

O'Rourke accused Warren of being "punitive" on certain issues regarding wealth and entered into a heated exchange with South Bend, Indiana, Mayor Pete Buttigieg over guns.

The moments echoed some that O'Rourke had at earlier debates, suggesting that the Texas Democrat who entered the presidential race with sky-high hopes believes what he has been doing will be enough to land him on the debate stage again.

The wild card of the night in this group of bubble Democrats was Gabbard, who was back on the stage after failing to qualify for September's debate.

Gabbard, a Hawaii congresswoman and Iraq War veteran, was aggressive on Tuesday, regularly closing her questions by asking other candidates directly to join her in certain policy proposals that she knows are out of step with others in the field.

The most contentious moment came when she argued that the violence in Syria at the hands of Turkey was, in part, to blame on Washington politicians who "who have supported this ongoing regime-change war in Syria that started in 2011." Gabbard said at one point that "politicians in our country from both parties" have "the blood of the Kurds" on their hands.

Buttigieg, the only other veteran onstage, aggressively attacked her answer, calling her "dead wrong."

For Gabbard, though, the moment could be viewed as a win for her devout following, who, for the most part, are anti-interventionist.

Still, Gabbard appears to be preparing for a world in which she is not on the debate stage. Days ahead of October's debate, she had threatened to boycott the contest in protest of the DNC rules, seemingly setting up grounds for her to complain if she doesn't make the November contest.

Gabbard's campaign did not return a request for comment.