Since the US enacted a federal income tax over a century ago, people making higher incomes have had to pay higher marginal income tax rates than those with lower incomes.

But at the Republican presidential debate Wednesday, Dr. Ben Carson— the current runner-up in national and early state polls — referred to progressive taxation as "socialism," adding that it "doesn't work so well."

Most Republicans don't believe that taxes on the rich should be increased, but mainstream figures in the party are usually far more careful in their rhetoric than Carson was. His statement suggests that we've had a socialistic tax system for over 100 years.

Then, poll frontrunner Donald Trump pushed back with a response that revealed his own divergence from his party's line — indeed, it could have been delivered by a Democrat like Barack Obama.

"We've had a graduated tax system for many years," Trump said. "It's not a socialistic thing." He went on to say that his own tax plan would be "a major reduction for the middle class," and that "the hedge fund guys" would pay more. "I know people making a tremendous amount of money and paying virtually no taxes," he said, "and I think it's unfair."

Trump and Carson's exchange on taxes emphasizes that they are both political outsiders

Carson had been asked what he thought about Donald Trump's advocacy — quite unusual for his party — that wealthy people should pay more in taxes. "Donald Trump believes in progressive taxation," moderator Jake Tapper said. "He says it's not right that rich people pay the same as the poor." Asked to tell Trump why he thought he was wrong, here's what Carson said:

It's all about America. You know, the people who say, "The guy who paid a billion dollars because he had 10, he's still got $9 billion left? That's not fair. We need to take more of his money!" That's called socialism. That doesn't work so well. What made America into a great nation was the fact that we said "That guy just put in $1 billion, let's create an environment that's even more conducive to his success, so that next year he can put in $2 billion." And that's the kind of thing that helps us to grow. We can't grow by continuing to take a piece of pie and dividing it and redistributing it.

The retired neurosurgeon's response shows his willingness to take far-right positions. Party leaders like Mitt Romney have long sought to cut taxes (including for the wealthy) and to reduce the number of tax brackets (flattening the tax code). Yet a totally flat tax, or deeming the progressive income tax as a whole "socialism," has generally been the territory of the further right.

That's likely because, in polls, most Americans express views much more similar to Donald Trump and Barack Obama's — the middle-class should pay less, and the rich should pay more. Advocating against the progressive income tax, and for a totally flat tax, could alienate these voters.

For his part, Trump not only says the wealthy should be taxed more, but has pledged to protect Social Security and Medicare spending too. And there's some evidence that his views are quite popular among Republican voters. A 2014 Gallup poll found that 45 percent of Republicans thought the wealthy pay too little in taxes, compared with just 21 percent who thought they pay too much. That's why, as I wrote on Tuesday, Trump's candidacy is such a threat to the party's elite-driven orthodoxy on economics.