Want to know why Jeb Bush’s donors are getting nervous? Consider this one number, calculated from his just-released third quarter Federal Election Commission financial filing: 85 percent.

That’s Bush’s “burn rate," the money he spent compared to the money he took in. Not saving much for the future, is he? His campaign raised just over $13 million for the third quarter, but it spent $11 million. If it were a public for-profit business, numbers like that could cause its stock to tank.

No wonder Bush has moved to cut staff salaries and reduce other expenses. From the beginning, he concentrated on building a full-service, front-runner campaign. But that’s not the position he is now in.

“Bush’s campaign once saw its size and staff as its strength. But the newly released campaign finance reports indicate it could be a liability if fundraising slacks further,” notes Politico’s Marc Caputo this morning.

Burn rate per se isn’t really Bush’s problem. You know who else spent a comparable percentage of their third-quarter inflow? Hillary Clinton. Hers was even higher, as she doled out close to 90 percent of the $28 million she raised from July through September.

She’s got 78 paid organizers in Iowa and 50 paid staffers in Iowa. She travels almost exclusively by private jet. Mrs. Clinton’s not bumping around early voting states in a white van and crashing at supporters’ homes.

The difference, of course, is the size of your cash cushion and future financial prospects. Clinton has $32 million cash-on-hand, while Bush has $10 million. Clinton’s fundraising prospects remain strong – she’s 20 points up in the polls, and donors always find front-runners appealing. Bush’s prospects... well, he’s tied for sixth at the moment, with 7.3 percent of the GOP vote, according to RealClearPolitics. He’s falling far short of donors’ early expectations.

The most interesting comparison here for Bush isn’t Clinton, but perhaps Marco Rubio. It’s Rubio that pundits consider Bush’s main rival for the position of most-electable non-Trump candidate in the GOP field.

In some ways Senator Rubio is no better off than his fellow Floridian. He only raised about $6 million in the third quarter. Yet he spent $4.6 million. His burn rate was 80 percent – close to Bush’s.

Rubio has $9.8 million in cash-on-hand, also close to the Bush number.

But Rubio is higher in the polls, at 9.7 percent. He’s the top non-Trump, non-Ben Carson candidate. In other words, he’s the top non-outsider.

The bottom line: This means sniping between the Bush and Rubio camps will intensify. Only one of them is likely to make it through the early primaries and emerge as a contender around which the GOP establishment can rally.

“They’re both competing for the same donors, and both are felling pressure to show progress. And frankly, they’re both stuck in the second tier right now,” wrote NBC’s Chuck Todd and Carrie Dann this a.m.

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That’s why Jeb Bush Jr. on Thursday took a shot at Rubio’s Senate record, saying that the latter is missing so many floor votes by running for president that he should “drop out or do something."

Look for the Rubio camp to respond to this in kind.