By Michelle Conlin and Grant Smith

NEW YORK (Reuters) - Half a dozen Republican presidential candidates are edging toward financial crisis, raising the specter that some may be forced to drop out of the sprawling field of contenders.

They all spent more than they took in during the third quarter, according to campaign finance reports filed on Thursday. The six are: Kentucky Senator Rand Paul, former Arkansas Governor Mike Huckabee, Louisiana Governor Bobby Jindal, former New York Governor George Pataki, South Carolina Senator Lindsey Graham and former Pennsylvania Senator Rick Santorum.

Together, they raised $6 million but spent more than $9.5 million during the summer on everything from postage to travel to campaign rallies. All six are trailing badly in the polls.

"They are living on the edge," said Lawrence Noble, former general counsel to the Federal Election Commission."We are getting close to the time when a lot of these candidates are going to say, 'We can't do it, it can't be done,'" said Noble, now a senior attorney with the Campaign Legal Center, a campaign finance non-profit.

Campaigns have tipping points: the moment when a candidate does the math and realizes that he does not have enough money on hand or the prospect of more money from donors to stay in the race. One telling sign is the "burn rate" - jargon for how much a candidate spends versus how much he is raising. If the burn rate is high and donor enthusiasm low, then trouble ensues.

The math is simple, said Austin Barbour, who ran Rick Perry’s fund-raising Super PAC before the former Texas governor dropped out. Barbour is now a senior adviser to the campaign of former Florida Governor Jeb Bush.

When direct donations to campaigns are lackluster, as in the case of these six candidates, there may not be enough money to cover basic operating costs like travel, staff salaries and office equipment. Those costs are not typically covered by big money Super PACs, which are supposed to operate independently of the campaigns.

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“It’s really tough to survive with such little money,” Barbour said. “It puts a lot of pressure on a campaign because no one wants to put their candidate in debt.”

DANGER ZONE

The third quarter reports show the challenges. Any burn rate over 100 percent is considered dangerous by campaign finance experts. Pataki’s was 226 percent, Graham 188, Paul 181, Jindal 144, Huckabee 110 and Santorum 101.

Of those, Paul and Graham have the most money in the bank, with $2.1 million and $1.7 million respectively, while the rest are money-challenged. Pataki, for instance, had less than $14,000 on hand as of Sept. 30, less than the $17,600 billionaire candidate Donald Trump spent on yard signs in the third quarter alone.

The campaigns dismissed the suggestion they were in financial trouble.

Rand Paul's campaign stressed it still had the $2.1 million on hand. A Pataki staffer said his burn rate was just the “cost of a campaign for President.” And the Huckabee campaign said their candidate was experienced at running campaigns on shoestring budgets.

Spokesmen for Santorum, Graham and Jindal did not respond to requests for comment.

To be sure, tight budgets at this point in the race do not mean the campaigns are doomed. A candidate could have a breakout moment, like former Hewlett-Packard CEO Carly Fiorina, whose fundraising soared after a good debate performance. Candidates can also lend themselves money, as Democratic frontrunner Hillary Clinton did when she ran low during the 2008 White House race.

But the Republican candidates are bedeviled by another math problem. There are 14 Republicans vying for their party's nomination for the November 2016 election, more than double the number at this point during the 2012 election.

“The Republican field is way too large, there simply isn’t enough money to go around,” said Noble.

Small donors are the lifeblood of any campaign and candidates will live or die by their ability to tap into a broad base of supporters willing to contribute up to the maximum of $2,700.

Retired neurosurgeon Ben Carson, who is one of the front-runners in the Republican race, reported nearly 22,000 donors in the last quarter who have given more than $200 so far in the campaign. Bush had 7,300.

In contrast, Pataki had fewer than 80 donors last quarter; Jindal had under 300; Graham had nearly 650; Santorum under 300, and Huckabee more than 800. Among these five, Paul had the most, with more than 3,500.

The candidates could conceivably win the patronage of a millionaire or billionaire, who could funnel unlimited amounts of money into their Super PAC. But these fund-raising groups are prohibited from carrying out certain campaign activities and are therefore of limited help.

For example, Wisconsin Governor Scott Walker had a Super PAC with money in the bank, but after burning through $6 million in three months his campaign's coffers were bare and he was forced to drop out in September.

Perry, who quit the same month, hemorrhaged money and ended the third quarter with just $45,000 on hand. His Super PAC returned $13 million to its donors.

(Reporting By Michelle Conlin, editing by Paul Thomasch and Ross Colvin)