Is Ben Carson a real front-runner?

It's been a very good couple of weeks for presidential hopeful Ben Carson.

Despite - or perhaps because of - a third straight debate performance in which the retired neurosurgeon practically disappeared into the scenery thanks to his low-key public persona, he has continued to climb in the polls.

The latest survey, by NBC News, puts him at 29% support, six points up on the former consensus leader, Donald Trump.

Mr Carson also has the highest favourability ratings of any candidate, and he's the first or second pick of half the poll respondents - well ahead of all his competitors.

By all accounts, then, Mr Carson is on the verge of assuming the mantle of Republican front-runner. While Florida Senator Marco Rubio was the talk of the Washington establishment after his most recent debate performance, the soft-spoken doctor continues to be a campaign juggernaut.

But is Mr Carson's lead a mirage? Here are a few possible dark linings that could appear on Mr Carson's silver clouds.

Ben Carson attributes controversy over his comments to media obsessed with political correctness

Questionable comments

Mr Carson has compared gun control, political correctness, the US under Barack Obama and the progressive movement in general to Nazi rule.

He's also drawn slavery analogies on the issues of healthcare reform and abortion and said prison sex is evidence that homosexuality is a choice and Muslims should be disqualified from the presidency.

It's Mr Carson's particular gift that he can deliver lines like these in the calm voice of physician informing his patient that the situation is dire but that a cure is possible, and then allow any resulting wave of controversy to wash over him harmlessly.

Although Mr Carson is currently riding high, at some point the cumulative effect of line after inflammatory line could be too much for Republican voter concerned about his prospects in a general election. Or, perhaps, it could put a hard cap on the level of support Mr Carson is able to accrue, creating difficulties for him as the field eventually narrows.

Unlike Donald Trump, Ben Carson has yet to respond to calls for greater policy details

Policy holes

Mr Carson's differences with fellow front-runner Donald Trump are more than just ones of style. After earlier criticisms that he was running a campaign without substance, the New York billionaire has released a series of plans - on immigration, tax reform, gun control and veteran's affairs.

Other candidates have likewise released their plans, in greater and lesser detail.

As for Mr Carson? Not so much. His website features a grab-bag of proposals, lightly sketched, including a call for a constitutional amendment mandating a balanced budget, the "defeat" of federal education bureaucracy, the continued operation of the detention facility in Guantanamo Bay, Cuba, and healthcare reform based on tax-free personal health savings accounts.

He's also called for a flat tax on income that would replace the current progressive tax rate structure and has suggested that the rate be set at 10%, although it could range up to 15% depending on revenue necessities.

As a front-runner will Mr Carson be pressed to flush out more policy details? Mr Trump was. So far, however, the retired neurosurgeon has gotten a pass. That, however, might be changing.

"I want more details and answers," Doug Holtz-Eakin, president of the conservative American Action Forum, told the Street's Valerie Young. "We've seen what uncertainly does to the economy. So if he wants to genuinely have a good performance, he needs to lay it out so people know what to expect."

The Carson campaign has generated big fundraising totals - but at a high cost

Fundraising concerns

When third quarter fundraising totals were announced last month, Mr Carson made headlines with a $20m - which put him ahead of all Republicans and just behind Democrats Bernie Sanders and Hillary Clinton.

As the Atlantic's David Graham points out, however, those numbers conceal a less rosy reality. Although Mr Carson is posting big numbers, he's spending a lot to do so. Fifty-four cents of every dollar the neurosurgeon brings in is spent on fundraising - such as developing donor files, printing direct mail literature and paying postage.

By contrast, Mr Sanders - who relies heavily on online donations - spends just 4 cents of every dollar on fundraising.

Conservative columnist Erick Erickson, notes that Mr Carson spent 69% of the money he took in - his "burn rate", as it's called - and offers a grim prognosis.

"Carson's actual expenditure list reads like a wealthy Republican getting played by consultants," he writes. "I suspect there are some who see Carson as a cash cow."

While that could be great news for political operatives involved, it's not good for a candidate who wants to win elections.

Ben Carson's supporters have been devoted - so far

State polling

Although Mr Carson has shot to the top of several national polls, the Republican race for the presidential nomination is going to be decided on state-by-state battlegrounds, and his polls here so far have been less impressive.

Mr Carson has led in several Iowa polls, but the latest puts him in effectively in a tie with Mr Trump.

In New Hampshire - less fertile ground for an evangelical-backed conservative like Mr Carson - Mr Trump has posted numerous double-digit leads. The New Yorker has also dominated South Carolina and Florida surveys.

None of this is to say that Mr Carson can't turn his national support into a lead in key early-voting states. The reality, however, is that Mr Trump still poses a formidable opponent with loyal supporters who have stuck with him for months now.

Will Mr Carson consolidate his gains the same way? Time will tell.

Ben Carson will likely face greater scrutiny over his ties to a controversial nutritional supplement company

Mannatech controversy

During the third Republican debate in Colorado last week, moderator Carlos Quintanilla questioned Mr Carson about his ties to the nutritional supplement company Mannatech, which has paid millions of dollars in fines for deceptive marketing practices.

Mr Carson denied having "an involvement" with the company, saying only that he happily takes the product and was paid to give speeches for them. Quintanilla tried to press the issue but was booed by the debate audience, prompting Mr Carson to smile and conclude, "See, they know".

The questions about Mannatech can't easily be written off as a liberal-media obsession, however. The Jim Geraghty of the conservative National Review has been one of the most dogged investigators into the ties between Mr Carson and the supplement company, and after the debate he said Mr Carson's answers were "bald-faced lies".

"Mannatech wanted to improve its image and happily paid Carson, one of the country's greatest neurosurgeons, the man Cuba Gooding Jr played in the [TNT] movie - to appear at their events and to appear in the company videos," he writes. "Carson's lack of due diligence before working with the company is forgivable. His blatant lying about it now is much harder to forgive."

Mr Carson has pushed back, telling SiriusXM's Breitbart News Daily radio show that the National Review has become a "political tool" of his one of his Republican opponents and that the story was a "submarine that's sent by them".

Whether he's wrong or right about his allegations, chances are the Mannatech story isn't going away anytime soon.

Jonathan Chait questions Ben Carson's sincerity

Commitment questioned

Some writers, on the left and on the right, have attempted to draw lines between the aforementioned concerns - the litany of publicity-generating controversial statements, the lack of policy details, the questionable fundraising practices, the ethical questions surrounding ties to Mannatech - to draw larger conclusions about the nature of Mr Carson's presidential quest.

Leon Wolf, a diarist for the conservative RedState blog, wonders whether Mr Carson's apparent lack of command of the issues reflects poorly on his commitment to the campaign

"I am distressed that he has spent apparently no time in study educating himself on the basic issues he would face during the course of the run for his presidency," he writes. "I am 100% confident Carson could understand them very easily, which is precisely the reason I'm so bothered that he doesn't. It goes to the seriousness with which he takes the job of running for president, and his personal work ethic at that job."

He contrasts Mr Carson with former technology company executive Carly Fiorina, who has spent considerable time honing her command of the issues.

"I'm at a loss as to what Ben Carson has been doing at all, and I'm disappointed that he apparently doesn't take running for president seriously," he concludes.

New York Magazine's Jonathan Chait concludes that Mr Carson says the obvious explanation for Mr Carson's actions is his goal isn't the presidency, it's the accumulation of wealth.

"Carson is doing a lot of things that seem puzzling for a presidential campaign, but quite logical for a brand-building exercise," he writes. "Perhaps it is a giveaway that the official title for Armstrong Williams, the figure running the Carson 'campaign', is 'business manager', as opposed to 'campaign manager'. It does suggest that Carson is engaged in a for-profit venture."

The nebulousness of Mr Carson's campaign so far leaves him open to these sorts of attacks. How Mr Carson responds - with substance or bluster - will go a long way in revealing whether his candidacy can make it to the finish line.