Donald Trump and Paul Ryan. Zach Gibson/Getty Images

The group behind a brutal review of President Donald Trump and Republican leaders' framework for overhauling the tax code has hit back at suggestions that its study amounted to "propaganda."

The analysis from the Urban Institute and Brookings Institution's Tax Policy Center found that the plan would disproportionately benefit wealthier Americans and could end up increasing the tax bills of many middle-class Americans.

Republicans in both the House and the Senate have attacked the report, calling it skewed and incomplete since it did not analyze actual legislation — so far the administration has released only an outline.

"Their analysis was a work of fiction that Stephen King would've been proud of," Rep. Kevin Brady, the chairman of the House Ways and Means Committee, told Fox News Radio.

Mark Mazur, a director at the Tax Policy Center, defended the analysis to Business Insider in an interview on Wednesday.

Mazur said the TPC decided to come out with analysis at the early stage because "there is a demand for information on the unified framework" and because the goal of the TPC is to help provide as much information as possible. He said TPC researchers were careful to base all assumptions on existing ideas from the GOP leadership.

"Obviously there weren't all the details there, but we used the House Republican leadership 'Better Way' tax plan and elements of the Trump administration's proposal to fill in the blanks," Mazur said, referring to a plan proposed by House Speaker Paul Ryan.

Even with the assumptions, Mazur argued that the broad thrust of the report was correct.

"Where we came out on the results — that the tax plan as it stands right now would lose a lot of revenue and have the benefits tilted toward high-income taxpayers — those are directionally correct," Mazur said. "And I have not seen any serious dispute of that fact."

As Mazur noted, both the left-leaning Center for Budget and Policy Priorities and the Institute on Taxation and Economic Policy have come to similar conclusions as the TPC.

Republicans also attacked the TPC and Urban Institute as fundamentally biased.

"It's very predictable coming from this group," Ryan said. "I think The Wall Street Journal got this right when they said this is an anti-reform, propaganda group. It's anti-tax-reform."

Mazur pushed back on those assertions.

"First, Tax Policy Center has been scrupulously nonpartisan for a number of years, since it was founded in 2002," Mazur said. "Second, we're all in favor of doing real tax reform that makes the tax system more efficient, fairer, and simpler. And what we did in this analysis is point out some areas where the framework may have fallen short in those areas."

Mazur said he had not been contacted by any GOP lawmakers or their staff but would be open to discussing the proposal with those leaders.