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Oct. 20, 2015: This story has been corrected.

Grassroots givers are playing a pivotal role in shaping the 2016 presidential race, boosting insurgent candidates in both the Democratic and Republican primary contests as they challenge establishment favorites in the polls and the race for campaign cash.

Small-dollar donors who gave $200 or less accounted for about 40 percent of the approximately $144 million raised between July and September by the two dozen Republicans and Democrats running for president, according to a Center for Public Integrity review of campaign finance documents filed Thursday.

So who are the candidates raising the largest portion of their campaign funds from these people-powered fundraising machines? Those bucking their parties’ establishments and touting their desires to challenge the status quo in Washington.

A staggering 77 percent of the $26.2 million Bernie Sanders, an independent U.S. senator running as a Democrat, collected during the third quarter came from contributors giving $200 or less. The haul helped Sanders narrow the fundraising gap between his campaign and that of Democratic Party frontrunner Hillary Clinton, who raised $29.9 million during the same period.

Meanwhile, such small-dollar donors accounted for about 70 percent of the $3.9 million raised by Republican Party frontrunner Donald Trump, the billionaire real estate mogul and reality TV celebrity.

And they accounted for about 60 percent of the $20.8 million raised by Republican Ben Carson, a former neurosurgeon who collected more campaign cash during the third-quarter than any other GOP White House contender.

These outsider candidates’ small-dollar fundraising success reflects “grassroots unhappiness with the powers that be,” said Michael Malbin, executive director of the nonpartisan Campaign Finance Institute, which tracks money in politics.

“The strongest small donor campaigns are not about raising money,” Malbin added. “They’re about raising enthusiasm and getting actions.”

David Keating, president of the Center for Competitive Politics, a nonprofit that advocates for fewer campaign finance regulations, noted that small-dollar donors can add up to “serious money” over the course of a campaign.

“Dollar for dollar, small-dollar donors are worth more than big-dollar donors,” Keating said. “Those are the donors that can keep giving over and over.”

Keating added that such donors are also likely to turn out to vote for a candidate, and possibly volunteer as well.

As small-dollar conservative donors help boost the likes of Trump and Carson, the Republican Party establishment has not yet consolidated around a single choice.

Republican Jeb Bush, the former governor of Florida long believed by many to be the likely nominee in 2016, raised $13.4 million during the third quarter. That brings his total haul for the year to $24.8 million — not including the more than $103 million raised by Right to Rise USA, the super PAC Bush founded and raised money for before officially launching his campaign.

Bush’s money hasn’t kept others from challenging him for supremacy among the party’s donors.

Fellow Floridian Marco Rubio, for one, has eclipsed Bush’s campaign fundraising haul, raising $25.3 million for his presidential campaign, including money he transferred from his Senate campaign and a $5.7 million take during the third quarter.

And Sen. Ted Cruz of Texas, another outsider candidate, is also in the thick of it, raising $26.6 million so far this year, including $12.2 million between July and September.

All the while, many Republicans are still waiting for the field to narrow further before they pick a favored candidate, a Center for Public Integrity analysis of campaign filings and interviews with top donors and fundraisers show.

For instance, GOP megadonors such as casino magnate Sheldon Adelson and hedge fund manager Paul Singer remain on the sidelines of the race.

Likewise, Arkansas businessman Warren Stephens is still being courted by campaigns.

“I haven’t really settled on anybody,” he said, despite the fact that he’s donated this year to super PACs supporting the presidential bids of Walker, Bush and New Jersey Gov. Chris Christie.

So are Joe and Marlene Ricketts, the heads of the family that owns the Chicago Cubs baseball team. The couple was among the biggest supporters of Wisconsin Gov. Scott Walker before he dropped out of the GOP presidential race last month.

Nearly three-quarters of the nation’s top 100 political donors during the past six years — as identified by the Center for Responsive Politics — have contributed to presidential candidates or efforts supporting their bids, according to an analysis of campaign finance data by the Center for Public Integrity.

Among the biggest Republican donors, that’s essentially unchanged since mid-summer, a sign few were lured off the bleachers and into the game over the past three months.

Of course, not all of the candidates in the sprawling field have successfully stirred up financial support, and several campaigns reported running dangerously low on cash.

Several candidates reported spending more money during the third quarter than they raised.

One example: Kentucky Sen. Rand Paul, a Republican, brought in $2.5 million but burned through $4.5 million. Paul’s campaign reported having $2.1 million in cash on hand at the end of the quarter.

In addition, 11 active candidates each had less than $1 million in cash left in the campaign till as of September 30 — although one of them is Trump, who has said he is willing to self-fund his campaign as needed.

A dwindling stash of hard campaign cash played a significant role in forcing both Walker and former Texas Gov. Rick Perry out of the Republican primary race. Their withdrawals came despite billionaire backers willing to invest millions in super PACs supporting them, showing wealthy outside groups are not enough, on their own, to save a faltering bid.

Derrick Robinson, a spokesman for ActBlue, a Democrat-aligned fundraising outfit, told the Center for Public Integrity that online technology would help engage millions of small-dollar donors, even in the age of big-money super PACs, which have no contribution limits.

“There’s no question, small-donor giving is on the rise — even as super PACs have gained in popularity,” Robinson said.

Democratic consultant Joe Trippi, who served as Howard Dean’s campaign manager in 2004 when the former Vermont governor unexpectedly surged by galvanizing the grassroots, likewise agreed that technological changes have aided politicians.

But Trippi stressed that insurgent candidates still face uphill battles.

“We scared the daylights out of the establishment, but that was about all we were able to do,” Trippi said. “Both party establishments are pretty good at making sure that a candidate who isn’t of the establishment doesn’t make it.”

Ben Wieder contributed to this report.

This story was co-published with NBC News.

Correction, Oct. 20, 2015, 1:18 p.m.: Due to a data processing error, an earlier version of this story incorrectly reported the number of major political donors who have donated to the campaigns of presidential contenders and efforts supporting their bids. That number is nearly three-quarters, not half, as was originally reported.

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Michael Beckel reported for the Center for Public Integrity from 2012 to 2017.

Carrie Levine joined the Center for Public Integrity in October 2014 as a federal politics reporter investigating...