Even Ben Carson has a hard time believing he could be president.

"Probably in terms of the applying for the job of president, a weakness would be not really seeing myself in that position until hundreds of thousands of people began to tell me that I needed to do it," he said at the top of the Republican presidential primary debate on CNBC Wednesday night.

That explains why Carson managed to fade into the background despite the fact that he's running first in Iowa and second in most national polling. It explains why the moderators and Carson's Republican rivals all but ignored him. And it explains why Carson was ill-prepared to talk about the federal budget and other policy matters in a serious way.

Carson was in the middle of the stage Wednesday night. But he was nowhere near the center of the debate.

It should have been his breakout moment — a time for moderators to focus on his rise in the polls and for him to make a more comprehensive case for himself to Republican primary voters. Neither happened. Instead, he spoke for barely more than seven minutes. His big name check from a rival came when Donald Trump mentioned him in closing remarks.

It's easy for fellow Republican candidates and political analysts to dismiss Carson because his amiable nature and impressive personal narrative simply don't compensate for his failure to make a plausible argument that he would be better than any of the other candidates — much less all of them — as president. Despite his poll numbers, they're treating Carson as a non-factor. And that's about right.

Carson had a big chance Wednesday night to show that he's Oval Office material. But instead of stepping up, he demonstrated why it's so hard for the political world to see him as a viable candidate for the presidency.

His budget plans are a "fantasy"

It will be hard for Carson to convince Republicans that he's the best choice to cut spending and taxes until he proves he's got a handle on the federal budget.

CNBC's Becky Quick noted that his call for a 10 percent flat tax would reduce federal revenue by well more than $1 trillion. When Carson countered that he'd probably end up with a 15 percent flat rate, Quick pointed out that would leave a $1.1 trillion revenue gap and either require slashing government programs deeply or running a deficit.

"That's not true," Carson protested. Economic growth would make up the difference, he insisted.

Yeah, right, said former House Budget Committee Chair John Kasich (R-OH), who balanced the federal budget with President Bill Clinton in the 1990s. Kasich, who is now Ohio's governor, jumped in to accuse Carson of creating a "fantasy" with his numbers. Kasich may not be the most popular candidate among Republicans — and he took a risk by going after the well-liked Carson — but his knowledge of the federal budget is unassailable.

Carson, on the other hand, seemed to argue at one point that there's not much difference between $2.7 trillion he says his tax plan would produce in revenue and the $3.5 trillion he says the government spends now (it's closer to $4 trillion this year). Even in Washington the $800 billion difference between $3.5 trillion and $2.7 trillion is a lot of money.

Most viewers don't have federal budget numbers at their fingertips, but a serious presidential candidate should.

Carson would solve drug pricing issue by de-regulating

There are a few good answers to the question of how the government should respond, if at all, to a drug company charging $750 per pill for an AIDS drug that recently cost as little as $13.50 per pill and could be produced for as little as $1 per pill. Democrats have their own solutions, including Vermont Sen. Bernie Sanders's effort to tie the controversy to an ongoing congressional probe into drug pricing.

For Republicans, there's a pretty simple response: The government shouldn't interfere in the private market, which will correct for price gouging. There's a pretty good exhibit A for that line of argument in the emergence of a competitor that says it undercut the maker of Daraprim. But Carson argued Wednesday night that government regulation is the cause of price spikes like the one for Daraprim.

"Have these companies gone too far?" CNBC's Jim Cramer asked. "Should the government be involved in controlling some of these price increases?"

Here's how Carson responded:

Well, there is no question that some people go overboard when it comes to trying to make profits, and they don't take into consideration the American people. What we have to start thinking about, as leaders, particularly in government, is what can we do for the average American? ... So what we're going to have to start doing instead of, you know, picking on this group or this group, is we're going to have to have a major reduction in the regulatory influence that is going on. The government is not supposed to be in every part of our lives, and that is what is causing the problem.

Chris Christie jumped in a few moments later to offer a prescription that required no new regulation from the government but also took into account the possibility that a sudden spike in drug prices could be related to bad behavior in the private sector.

"We have laws already. We don't need newer laws," he said. "So what we do, though, is, if there is somebody who is price gouging, we have laws for prosecutors to take that on. Let's let a Justice Department — and I will make an attorney general who will enforce the law and make justice more than just a word. It will be a way of life."

Carson backtracked on at least one of his own positions

There was a point at which Carson wanted to cut federal subsidies for oil companies and redirect the money to ethanol. That position is very popular in Iowa, the ethanol powerhouse that holds the first presidential nominating contest in February. And it's probably one of the many reasons he's now in first place in Iowa and has through-the-roof approval ratings among the state's Republican voters.

But subsidizing ethanol is a big no-no among economic conservatives who want to end government handouts for big business. So when he was asked about the redirection from bolstering oil companies to boosting Iowa growers, Carson backtracked.

"I was wrong about taking the oil subsidy," he said. "I have studied that issue in great detail. And what I have concluded, that the best policy is to get rid of all government subsidies and get the government out of our lives and let people rise and fall based on how good they are."

It's a position that could help Carson with national economic conservatives and in states outside of Iowa. But it's a risky proposition within the Hawkeye State, where Carson had a low double-digit lead in the last two major polls conducted.

Carson isn't a factor because he's not viable right now

It was evident in Wednesday night's debate that neither the moderators nor the other Republican candidates had much interest in tangling with a candidate who is both personally popular and unlikely to win the nomination.

Indeed, it's hard to see how a candidate with no political experience and a loose handle on public policy could win a major party's presidential nomination. For Carson to become a threat, he'll have to up his game — and fast.

Otherwise, he'll have to rejoin the rest of the political world in thinking his biggest weakness is that it's hard to envision him running the country.