Ben Carson, Jeb Bush and Ted Cruz appear to be leading the money race in the Republican presidential primary, according to third-quarter numbers released by the campaigns ahead of Thursday's filing deadline. 

Carson far out-raised his GOP rivals in the last quarter, bringing in more than $20 million, thanks to a surge of support in the polls and accompanying rush of small-dollar donations. 

While the retired neurosurgeon's numbers previously had been reported, Bush only released his fundraising details Thursday afternoon. Though there was speculation as to whether the one-time front-runner would be able to sustain the fundraising pace as his poll numbers have slipped, the former Florida governor's campaign says he raised nearly $13.4 million in the latest period, outpacing his second-quarter haul. 

Cruz, a Texas senator, followed with $12.2 million raised. 

Billionaire Donald Trump, the Republican front-runner, raised $3.9 million during the last quarter, but is not actively fundraising and is instead spending some of his own cash on the campaign trail. 

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The totals suggest Bush and Cruz, among others, have the financial backing to sustain an aggressive campaign, for now, even as Trump and Carson lead the polls. Bush reported having $10.3 million in the bank at the end of September, while Cruz reported $13.5 million on hand. While Florida Sen. Marco Rubio did not raise as much in the third quarter, he also reported a sizable, $11 million war chest. 

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But Carson's numbers are especially impressive in that he more than doubled the $8 million he made in the last quarter, raising $12 million in the month of September alone. 

"He is converting the populist appeal of his message into support and campaign dollars, which will allow him to continue to wage a viable campaign -- but whether this allows him to broaden his base of support remains to be seen, especially given the competition he faces from other non-traditional and conservative candidates," said Tony Corrado, professor of government and a campaign finance observer at Colby College in Maine. 

Carson had $11.5 million cash on hand in the third quarter, cementing his status as a top-tier GOP candidate, second only to Trump in recent surveys. 

The Federal Election Commission filing deadline is Thursday, though many of the campaigns decided to report their contributions to the press beforehand. The third quarter covers the period of June 30 through Sept. 30. 

Meanwhile, Trump told Fox News on Tuesday that he has spent "very little" on his campaign so far. "I've spent zero on advertising. Every [network] they have covered me a lot. It's almost like if I put ads on top of the program it would be too much, it would be too much Trump. I've spent the least money and have the best poll numbers."

The Democrats brought in a far greater take than the Republican candidates, in part because there are fewer of them vying for the party's donor base. According to front-runner Hillary Clinton's campaign, she raised $28 million during the third quarter and had $32 million on hand. Her main opponent, Vermont Sen. Bernie Sanders, raised $26 million and had $25 million in cash on hand. 

Carson apparently is spending the money at a quick pace, as he tries to sustain his momentum. According to his campaign, he spent $14 million from July to September. 

"We will be able to fully fund our most expansive get-out-the-vote program, and our most expansive advertising program and our most expansive social media program in Iowa through the caucuses," Doug Watts, his spokesman, told the Des Moines Register Thursday. 

Other candidates are pulling in substantial figures as well. 

Republican Carly Fiorina has benefited from a boost after solid performances in the first two primary debates. According to her campaign, she raised $6.8 million in the third quarter and had $5.8 million on hand, compared with the $1.7 million she took in during the second quarter from April to June. She joined the race officially in May. 

Rubio, according to his campaign, raised $6 million in the third quarter -- compared with $8 million in the second quarter -- and has about $11 million on hand. Political observers say he is competing for the same establishment donors as Bush, but could benefit from a shot in the arm by casino magnate Sheldon Adelson, who recent reports indicate has been warming to the Florida senator. Adelson spent a total of $92 million on political candidates and causes in 2012. 

The figures being reported by the campaigns this month pertain to campaign accounts, and not the super PACs backing them, which are pouring in millions more on top of what the campaigns are spending. 

Analysts say the ability to raise money through a large donor base, particularly from small donors, signals the long-term health of a campaign over all -- perhaps with the exception of self-funder Trump. 

In this arena, Carson appears to be ahead too, receiving more than 600,000 donations from more than 350,000 donors, his campaign reported to the press at the end of September. Meanwhile, Cruz's campaign said he had a total of 120,000 donors through the third quarter. Still, none of this compares to Sanders, who says he has had 1 million donors since the start of his campaign in late May. Clinton's people say 93 percent of her donations were $100 or less. 

Other Republican candidates are pulling in less money. 

Ohio Gov. John Kasich raised $4.4 million, spent $1.71 million and finished the quarter with $2.66 million on hand. New Jersey Gov. Chris Christie raised $4 million in the last quarter, spent nearly $3 million, and has about $1.4 million on hand. 

Kentucky Sen. Rand Paul raised $2.5 million, a decline from the $6.9 million raised in the second quarter, and had some $2 million in cash on hand. 

The Paul campaign sent out a memo Thursday defending its status in the race, saying some "are pushing a false narrative that Senator Rand Paul (SRP) is on the ropes." 

The campaign maintained that Paul "has the best organization in America." 

Louisiana Gov. Bobby Jindal reported raising $1.16 million in the third quarter. Former Virginia Gov. Jim Gilmore reported raising $62,000. Other Republican candidates have not yet announced their fundraising totals.

Fox News' Serafin Gomez and Mike Emanuel contributed to this report.