Washington(CNN) With the deadline fast approaching to qualify for the September debate stage, a handful of Democratic presidential candidates is on a spending spree to find donors and supporters who can help them meet the debate requirements.

The stakes are high for lower-tier candidates, hoping to improve their political fortunes with a breakout moment in a nationally televised debate. But they face barriers to entry: They must attract 130,000 unique donors and register at least 2% support in four polls endorsed by the Democratic National Committee.

California billionaire Tom Steyer, who recently announced he had met the 130,000-donor threshold, still had not met the polling requirement for the September debate as of Thursday morning. He's outspending the Democratic field, pumping nearly $4.7 million on Facebook and Google advertising alone, a CNN tally of data provided by the companies shows.

Steyer also has poured millions more into television ads to introduce himself to voters.

He shows no signs of slowing. In the last week, Steyer spent nearly $400,000 on Facebook ads to lead all candidates in advertising spending on the platform.

Other candidates who have yet to qualify also have ramped up their spending.

New York Sen. Kirsten Gillibrand this week launched a new 30-second ad -- part of a $1 million television advertising campaign in the early nominating states of Iowa and New Hampshire -- that touts her record in Congress and her stances on women's rights and health care. Gillibrand has not met either the fundraising or polling thresholds for the debate, despite her costly efforts to win over voters.

Gillibrand has relied heavily on digital advertising in her presidential quest, spending more than $3.2 million in Facebook and Google ads over the course of the campaign. Her online spending exceeds that of several other candidates in the first tier of the Democratic nomination battle, including Vermont Sen. Bernie Sanders and former Vice President Joe Biden.

A recent Facebook ad from Gillibrand deployed her young son Henry to ask for donations as small as $1.

One ad running from another senator, Colorado's Michael Bennet, tells potential supporters "I need your help ASAP" to meet the donor threshold: "Could you chip in to be a founding donor?" He's spent more than $571,000 on Facebook ads during the course of the campaign, the company's data shows.

Montana Gov. Steve Bullock -- a vocal critic of the debate qualifications -- has run a series of ads slamming the DNC and asking supporters to get him on the stage for the next round. In one ad, Bullock's campaign writes: "First: The DNC BLOCKED Governor Steve Bullock from the first debate. Then: Governor Bullock made the July DNC debate stage and showed the country why he's the BEST choice for President! But now: We could still be blocked from the next debate if 130,000 people don't donate." Bullock has spent more than $270,000 on Facebook and Google advertising.

'Begging for a dollar on Facebook isn't going to help'

Digital strategists say candidates should do more than ask for money if they hope to break through to donors and voters.

"If candidates are six or seven months into their campaigns, and they still are struggling to meet the donor threshold, begging for a dollar on Facebook isn't going to help," said Kyle Tharp of Acronym, a progressive non-profit group that tracks digital spending. "Those campaigns should reassess their message and core argument about why they'd be a better president than everyone else."

In an interview Tuesday on CNN, Gillibrand said she's hurt by not having national name recognition and acknowledged that she's also faced some backlash from the party's elite donors for her role in pushing for the 2017 resignation of then-Minnesota Sen. Al Franken, following allegations of sexual harassment. (Franken, a Democrat, has said recently that he regretted his decision to step down.)

"My view is this: It's not easy, but you have to do the right thing," Gillibrand said. "We should have the courage to do so."

In all, Democratic candidates have spent more than $38 million on Facebook and Google advertising so far during the 2020 campaign. Both firms began releasing details of their political advertising for the first time in 2018, so comparable 2016 data isn't readily available.

For candidates who just recently qualified for the debate, or are still trying, specific appeals for support in order to make the stage have consumed a significant share of digital advertising budgets. According to a digital ad tracking tool created by Bully Pulpit Interactive, a Democratic firm, multiple candidates have devoted more than half of their digital advertising over the last month to ads related to the debate.

From July 20 to August 10, for instance, Julián Castro, Cory Booker and Marianne Williamson each mentioned the debates in more than 70% of their digital ads. During the same period, Washington Gov. Jay Inslee and Gillibrand mentioned the debates in about half of their ads, the data show.

The next round of Democratic National Committee debates are Sept. 12 and Sept. 13 in Houston. In addition to the polling requirement, candidates must receive contributions from a wide geographical area. The donations from 130,000 individuals must come from at least 400 unique contributors in each of at least 20 states.

The candidates have until Aug. 28 to meet those benchmarks.

So far, 10 of roughly two dozen contenders have met the criteria: Biden, Booker, Castro, California Sen. Kamala Harris, Sanders, New Jersey Sen. Cory Booker, South Bend, Indiana, Mayor Pete Buttigieg, Minnesota Sen. Amy Klobuchar, former Texas congressman Beto O'Rourke, Massachusetts Sen. Elizabeth Warren and businessman Andrew Yang.

And even those who have made the debate stage are casting a wary eye on Steyer and the potential of his vast wealth, which Forbes pegs at $1.6 billion, to reshape the race. The former hedge-fund manager has committed to spending at least $100 million of his own money in the battle for the Democratic nomination.

In an email to supporters on Tuesday, Harris warned she's in danger of falling behind in the money race, noting the presence of a "billionaire hedge fund manager ... spending millions of dollars to buy his way onto the September debate stage."

Steyer spokesman Alberto Lammers said the California billionaire, who entered the race about six weeks ago, has spent heavily because "Tom got in there late, and we need to catch up."

Lammers said Steyer is gaining traction because voters like what he's saying and rejected the idea that Steyer was "buying" his way into the race, calling him a skilled "organizer," who knows his way around politics.

"His message is resonating. That's the bottom line," Lammers told CNN.

Steyer's previous political work has provided tangible benefits to his campaign. His Need to Impeach organization, created in 2017 to encourage Congress to launch impeachment proceedings against President Donald Trump, has collected more than 8 million names. His campaign has rented the Need to Impeach list, giving him a ready-made trove of potential donors.

Steyer now is one poll away from qualifying for the September debate.

There's still hope for candidates at risk of being elbowed off next month's debate stage.

In an email reviewed by CNN that was sent to a presidential campaign by the Democratic National Committee, the campaign was informed that qualifying polls for an October debate must have been released between June 28 and two weeks prior to the debate. That gives candidates who did not qualify in September additional time to make the next debate stage.