Republican presidential candidate Jeb Bush greets supporters in Knoxville, Tenn., earlier this month. (Wade Payne, AP)(Photo: Wade Payne, AP)

WASHINGTON — Republican Jeb Bush raised $13.38 million over the summer months, a dramatic slowdown from the torrid fundraising pace the former Florida governor set when he entered the presidential contest in June.

The haul puts Bush, the son and brother of former presidents, in second place so far behind one of the GOP field's rising insurgent candidates, retired neurosurgeon Ben Carson, who quickly scooped up $20.2 million over the summer as his campaign gained traction with religious conservatives. It's the latest sign that establishment-backed candidates are losing financial ground to political outsiders.

Three other Republican contenders — Carson, Texas Sen. Ted Cruz and Florida Sen. Marco Rubio — started October with bigger cash reserves than the $10.27 million Bush's aides said he still had available in the bank. Carson; Carly Fiorina, a former tech executive who's never held elective office; and Democratic contender Bernie Sanders, a self-described democratic socialist, all saw their fundraising soar over the summer.

Despite his political pedigree, Bush has lagged behind upstarts such as blunt-talking billionaire Donald Trump and Carson in early polling. A RealClearPolitics average of recent polls shows Bush in fifth-place among the 15 Republicans vying for their party's nomination, with Trump leading the field.

In a memo to supporters, Bush campaign manager Danny Diaz said the race has proved more "volatile" than expected. Few could have predicted "that a reality television star supporting Canadian-style single-payer health care and partial-birth abortion would be leading the Republican primary," Diaz said in a pointed dig at Trump.

Even so, the Bush campaign — with 37 paid staffers in the four early voting states and a massive database of potential voters — is built to last, Diaz said.

Bush had emerged as the GOP's fundraising leader when he entered the contest in mid-June, raising $11.4 million over a roughly two-week period, or more than $714,000 a day. That pace slowed to nearly $146,000 each day between July and September.

Bush's campaign announced his fundraising totals several hours before the deadline for federal candidates to disclose how they raised and spent campaign contributions during the July-to-September fundraising quarter.

Among the candidates who have already released their totals, Cruz collected $12.2 million; Fiorina raised $6.8 million; Rubio collected $6 million, Ohio Gov. John Kasich took in nearly $4.4 million and Kentucky Sen. Rand Paul raised $2.5 million.

Trump, who said he planned to self-fund his campaign, raised more than $3.9 million over the summer, much of it in unsolicited contributions from people drawn to his unorthodox candidacy. His aides said he took in nearly 74,000 donations during the three-month period, and only donated $100,779 of his own money during the quarter.

The initial fundraising reports show that top Democratic contenders are surpassing Republicans in direct campaign donations.

Right now, Hillary Clinton sits atop the 2016 fundraising field, collecting nearly $29.5 million during the July-to-September quarter, figures released Thursday evening show. Sanders was close behind at $26 million.

That doesn’t mean Republicans donors aren’t willing to spend big to win the White House.

Bush and others have been aided by super PACs that have already spent millions on TV ads to boost their candidacies. Details on how much those groups are raising won’t be public until the end of January — right before the Iowa caucuses.

In an effort to quell concerns about Bush's campaign fundraising, Diaz noted that a pro-Bush super PAC, Right to Rise, has reserved $33 million in TV advertising in Iowa, South Carolina and New Hampshire.

The 2016 campaign has demonstrated, however, that deep-pocketed super PACs can’t save faltering campaigns. Wisconsin Gov. Scott Walker and former Texas governor Rick Perry dropped out of the race recently, despite the backing of super PACs with big war chests. Perry raised only $287,000 over the summer months and had just $44,553 leftover in his campaign bank account at the end of September, according to a report filed Thursday.

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