Republican U.S. presidential candidate Dr. Ben Carson looks back as business Donald Trump (not pictured) takes the stage as he stands with former HP CEO Carly Fiorina, U.S. Sen. Ted Cruz and Gov. Chris Christie before the start of the 2016 U.S. Republican presidential candidates debate held by CNBC in Boulder, Colorado, Oct. 28, 2015. Reuters

Republican presidential candidates sparred early in the third GOP debate Wednesday over their tax plans, many supporting a flat tax rate as a replacement for current tax policies. Retired neurosurgeon Ben Carson was questioned repeatedly about the deficit that the CNBC moderators projected would ensue with his plan.

Carson defended himself, saying that the flat tax rate under his plan would be closer to 15 percent instead of 10 percent as the moderators said. He said that part of that tax plan would be paid for through cuts to government spending. The moderators noted that it would require a 40 percent reduction in government spending.

"We have 645 federal agencies and sub agencies," Carson said. "Anybody who tells me that we need every penny in every one of those is in a fantasy world. Also, we can stimulate, that's going to be the real growth engine: Stimulate the economy."

But others on the stage felt that it was Carson who was living in a fantasy world. "This is the fantasy that I talked about in the beginning," an exasperated Ohio Gov. John Kasich said when asked about Carson's tax plan. "These plans would put us trillions and trillions of dollars in debt."

While virtually all of the candidates promise tax cuts, several -- including Sen. Rand Paul, R-Ky., and Sen. Ted Cruz, R-Texas -- go further with the flat tax rate. Flat tax rates are favored by pundits like conservative Glenn Beck, who called Paul's plan "erotic." The Tax Foundation, a think tank that has been described as pro-business and conservative, has estimated that Paul's 14.5 percent flat tax on all income sources would grow the economy by 12.9 percent over time, create 4.3 million jobs and cost $1.8 trillion over ten years.

Opponents to flat tax rates point to the enormous income inequality rates in the U.S. Because the top wage earners make so much more money than the bottom wage earners, a flat tax rate would disproportionately benefit the wealthy, the argument goes. A flat tax would therefore also increase wealth inequality.

The third Republican debate was hosted by CNBC and was billed as a debate focused on the economy and money matters. National polls averaged together by Real Clear Politics show real estate mogul Donald Trump in first place with 26.8 percent of the vote and Ben Carson in second place. Neither of those two candidates have been elected to public office, adding to the peculiar nature of this campaign season.

Those two candidates are followed by Florida Sen. Marco Rubio (9 percent), former Florida Gov. Jeb Bush (7 percent), Cruz (6.6 percent) and former Hewlett Packard CEO Carly Fiorina (5.8 percent). None of the other candidates receive more than 5 percent support.

Carson and Rubio have been rising in polls since the last Republican debate. Carson has done so partly as a result of his debate performance, in which he seemed like a calm and soft spoken alternative to Trump’s bombastic style. Rubio was praised for his impressive foreign policy knowledge, which the second debate focused on.