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

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.

They all spent more than they took in during the third quarter, according to campaign finance reports filed on Thursday (Oct 15).

Together, they raised US$6 million (S$8.3 million), but spent more than US$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 Mr 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 Mr 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 realises 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 Mr Austin Barbour, who ran Mr Rick Perry's fund-raising Super PAC before the former Texas governor dropped out. Mr Barbour is now a senior adviser to the campaign of former Florida Governor Jeb Bush.

When direct donations to campaigns are lacklustre, 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.

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

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

Of those, Mr Paul and Mr Graham have the most money in the bank, with US$2.1 million and US$1.7 million respectively, while the rest are money-challenged.

Mr Pataki, for instance, had less than US$14,000 on hand as of Sept 30, less than the US$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.

Mr Rand Paul's campaign stressed it still had the US$2.1 million on hand. A Pataki staff member 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 Mr Santorum, Mr Graham and Mr 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 chief executive 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 Mr 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 US$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 US$200 so far in the campaign. Mr Bush had 7,300.

In contrast, Mr Pataki had fewer than 80 donors last quarter; Mr Jindal had under Mr 300; Mr Graham had nearly 650; Mr Santorum under 300, and Mr Huckabee more than 800. Among these five, Mr 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 US$6 million in three months his campaign's coffers were bare and he was forced to drop out in September.

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