Jeb Bush isn't the leading Republican candidate but recent campaign finance data shows he and Hillary Clinton are Wall Street favorites. (Reuters)

The Democratic presidential contenders dramatically outpaced their Republican counterparts in the race for campaign cash last quarter, spotlighting how the parties are taking divergent paths in their pursuit of 2016 funding.

The emphasis by Hillary Rodham Clinton and Bernie Sanders on raising money directly for their campaigns has helped them amass large donor pools critical to generating the estimated $1 billion each party’s candidate will need to raise by Election Day.

While GOP candidates put an intense focus early in the year on raising huge sums for independent groups, many have had less success in attracting smaller donations that are the lifeblood of campaign operations, campaign finance reports filed Thursday show.

The risk of the big-money approach was underscored by the modest $13.4 million raised over the summer by former Florida governor Jeb Bush, who earlier this year was expected to be a financial powerhouse. Just $877,000 came from donations of $200 or less.

After helping scoop up more than $100 million for his allied super PAC before declaring his bid, Bush is now struggling to explain to backers why he is not getting more traction among the party’s rank-and-file.

Explore the largest donors to 2016 presidential candidates

Two GOP contenders — former Texas governor Rick Perry and Wisconsin Gov. Scott Walker — have already dropped out of the race after running low on cash.

“You could have this big super PAC, but if you have limited momentum and limited money to keep the campaign going, it’s like the guy at the top of Mount Everest with two broken legs and an extra oxygen tank,” said Republican strategist Matthew Dowd. “You’re living longer, but you’re not going anywhere.”

One of the challenges for Bush and other GOP hopefuls has been the dominance of real-estate impresario Donald Trump, who has siphoned off much of the enthusiasm in the base. The businessman raised $3.8 million, even though he has pledged to self-fund his campaign and is not soliciting contributions.

“Donald Trump has basically stultified the fundraising for these candidates,” said Anthony Scaramucci, who had been Walker’s national finance co-chair and is now backing Bush. “He’s the Trump speed bump. His ratcheting up in the polls has made it very difficult for more establishment Republicans to get traction with donors.”

In all, six Democratic candidates reported raising $123.2 million for their campaign committees so far this year, while 15 GOP candidates pulled in $143.5 million overall.

Clinton and Sanders together had $60.1 million on hand at the end of September. Meanwhile, the 15 Republicans combined reported having $61.2 million in the bank.

How much money is behind each campaign?

The totals raised on each side by the candidate-aligned super PACs are unclear, because they do not have to report their fundraising until Jan. 31. But by mid-year, GOP allied groups had raised $234 million and Democratic groups had pulled in $17 million.

Retired neurosurgeon Ben Carson raised the most in the Republican field last quarter, scooping up $20.8 million. He still fell short of the totals posted by Clinton ($29.5 million) and Sanders ($26.2 million).

Other GOP candidates trail behind: Sen. Ted Cruz of Texas ($12.2 million), former Hewlett-Packard chief executive Carly Fiorina ($6.8 million), Sen. Marco Rubio of Florida ($5.7 million), Ohio Gov. John Kasich ($4.4 million), New Jersey Gov. Chris Christie ($4.2 million) and Sen. Rand Paul of Kentucky ($2.5 million).

One of the Democrats’ major structural advantages is the size of their small-donor bases. A staggering 650,000 individuals have given Sanders money already, while nearly 400,000 have donated to Clinton, her campaign said Thursday.

The senator from Vermont spent just $11.3 million last quarter, ending September with almost $27.1 million in the bank.

Clinton spent at a much faster pace, shelling out $25.7 million as her campaign made deep investments in building its data analytics and national ground operation. Her biggest expense was for a staff that swelled to 511 over the summer — $8.5 million went to payroll and taxes. She headed into October with nearly $33 million on hand.

Carson and Cruz appear to have built the strongest small-donation operations on the GOP side. Carson raised 60 percent of his money last quarter in low-dollar contributions — more than $12 million — thanks to a donor base of 402,000 people. Cruz brought in $5.3 million in small contributions, 44 percent of his total haul.

Fiorina also saw a late burst of small donations. Almost half of the $6.8 million she raised was in low-dollar contributions, with donations spiking after her two lauded debate performances.

Low-dollar contributors are key to a candidate’s long-term health — not just because they can provide repeated injections of cash, but because they bring energy.

“That’s the biggest leading indicator for us about who is hot,” said Becki Donatelli, a top GOP digital strategist whose firm is working for eight of the 2016 candidates. “These are your volunteers. They’ve got skin in the game.”

That has been a weakness for Bush, whose small donations amounted to just 6.5 percent of his total last quarter. Instead, his finance operation relies heavily on a network of longtime family loyalists. On Thursday, his campaign released a list of 342 fundraisers who have bundled at least $17,600 for his bid, but it did not indicate how much each person has raised. More than 70 of the bundlers are from his home state of Florida.

Bush raced through $11.5 million over the summer, spending money at a steep “burn rate” of nearly 86 percent.

He was not alone: Many of the candidates spent money at an intense clip in the third quarter.

Carson’s campaign spent $14 million between July and September. Of that, $9.5 million was spent on media and advertising. Nearly $2.7 million went to direct-mail expenses, and $1.4 million was allocated for payroll expenses.

Trump burned through $4 million in three months, more than the $3.9 million his campaign took in. He reported paying more than 50 consulting firms or individuals, including security guards, strategists, communications specialists, event producers, video­graphers, photographers, web designers and a host of administrative support staffers.

Trump’s biggest expenses appear to be the renting out large venues to host elaborate events that attract thousands, as well as campaign swag. He has thus far spent more than $800,000 on T-shirts, hats and other campaign-branded items. He also appears to have purchased radio airtime in Virginia and has hired telemarketing and direct-mail firms.

Paul spent $4.5 million, nearly double the $2.5 million he brought in — an unsustainable burn rate of 186 percent. His biggest expenditures were for campaign consulting and online advertising.

Louisiana Gov. Bobby Jindal (R), who has struggled to break out of the bottom tier of 2016 candidates since launching his bid in June, raised less than $600,000 during the third fundraising quarter in 2015. He spent more than $800,000, with the bulk of those expenditures going to payroll, media and campaign consulting.

Thursday’s filings shed light on the dire financial predicaments that led Walker and Perry to drop out.

Perry, a former Texas governor, raised just $252,000 in contributions during the most last recent reporting period — from July 1 to Sept. 30 — but racked up more than $1 million in operating expenditures during that same time. To sustain that pace, the campaign relied on the $884,000 it held at the beginning of the reporting period.

In all, the campaign paid nearly $300,000 in consulting fees on expenses ranging from finance to media specialists. Of that, $200,000 went to Abstract Communications LLC, a consulting firm registered under the name of his campaign manager, Jeffrey Miller. Miller could not be reached to comment.

Jose A. DelReal, Tom Hamburger and Jenna Johnson contributed to this report.