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Jeb Bush’s campaign people have a new worry.

It’s not Donald Trump — the unlikely lingering GOP front-runner has been a source of concern among the Republican presidential candidates for some time now.

It’s not even so much the other GOP candidates, Beltway outsiders Carly Fiorina and Ben Carson, who in recent weeks have generated buzz.

Bush supporters’ new concern is the former Florida governor’s one-time mentee and one-time friend, Sen. Marco Rubio, a fellow Floridian, who is rebounding in polls and attracting the interest of donors whom the Bush camp might have considered a slam-dunk at one time.

A new Wall Street Journal poll of GOP voters, for instance, shows Bush at around 7 percent, and trailing the three outsiders as well as Rubio. His latest poll number is half what it was in July.

The Bush camp is beating the but-we-have-a-big-war-chest drum to persuade supporters that their candidate has enough of a monetary cushion – $100 million – and a vast national operation to weather the many storms of a presidential run.

Bush campaign advisers are going out of their way to allay fears among supporters, and tell them that he’s still a candidate to beat, and is in the race for the long haul, Politico reports.

To buttress their argument, they cite data that is based on predictions and projects Bush in the top slot among GOP candidates, according to Politico. But the website notes that even this data now shows Rubio beating Bush and topping the list of GOP candidates.

“I don’t know if it’s panic or paranoia in Miami, but they are losing [Scott] Walker people to Marco, and if you say what’s true, they get mad,” said one Bush donor to Politico, which did not identify him upon his request. “I think it’s just reflective of what’s been going on for the past month or so and the way the race, at least in the establishment lane, has shifted. It’s really Jeb or Marco now. Marco’s fundraising has picked up, and Jeb’s has stayed flat.”

The Bush camp is beating the but-we-have-a-big-war-chest drum to persuade supporters that their candidate has enough of a monetary cushion – $100 million – and a vast national operation to weather the many storms of a presidential run.

“Our campaign has the scale to go through all the early states and into March and have the organization to get us on the ballot, to go get delegates and acquire delegates throughout the process,” said David Kochel of the Bush campaign to Politico.

But even in the fundraising department, Rubio has gained momentum, after Wisconsin Gov. Scott Walker’s withdrawal from the race and two strong GOP debate performances.

Politico quoted another unnamed Bush donor who ranked the anxiety level over Bush at a “6 or 7” on a 1 to 10 scale.

“This is one space that Jeb has owned,” said the donor about fundraising. “You can’t give up your position of strength if you need to build momentum in other places.”

Rubio's stock has been rising on social media too as his barbs directed at Trump have been praised as clever and effective.

The Washington Post noted: "Jeb’s efforts to challenge Trump failed to capture the imagination. Now, suddenly, the internet is abuzz with talk about how Marco Rubio is the one who is drawing blood from Trump, and talk of Rubio having a plausible shot at the nomination is on the rise."

Rubio’s people hope that donors who’ve been holding back on getting behind a particular candidate while poll results continue to fluctuate will start to move toward the first-term senator.

“There’s less money out there than people realize,” said Katie Packer Gage, whose firm is doing work for Rubio in Michigan, to Politico. “Lots of donors are holding their money because, if the big donor wants anything, they want to be with a winner, and they want the right candidate.”

Rubio is drawing enthusiastic crowds in Florida, most recently on Monday when he spoke at a rally there.

Carl Blankenship, a former state trooper who became impressed by Rubio after the second GOP debate, said that while he admired Bush, he felt it was time for a fresh face.

"I understand Jeb Bush did a good job for Florida as governor, and he is probably very capable. But I think we need a new voice," Blankenship said to the Tampa Bay Times. "It's time that a younger generation step forward to take up the reins, and I think Sen. Rubio is the perfect person."

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