Paul Ryan Win McNamee/Getty Images Republicans are taking on a brutal study of their new tax reform plan, slamming its assumptions and the conclusion that their framework would benefit wealthier Americans.

The study, released by the Urban-Brookings Tax Policy Center, found that the ideas contained in Republicans' nine-page tax reform document would disproportionately favor the wealthiest Americans, while some middle income Americans would see their taxes increase over the next 10 years.

Rep. Kevin Brady, chair of the House Ways and Means committee and one of the "Big Six" tax negotiators, said the TPC report was "misleading, unfounded, and biased."

"Their analysis was a work of fiction that Stephen King would've been proud of," Brady told Fox News Radio on Tuesday.

House Speaker Paul Ryan also brushed off the analysis during an appearance on Fox News.

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

Republicans argue that their plan is not yet completed, so any analysis of the plan is premature. The framework does avoid many specifics and contains vague language, which is designed to give the committees writing the legislation some leeway on specific provisions and avoid pressure from lobbyists.

"They're literally making up details, assuming what Congress will do, in order to come up with these kinds of results, to come up with these talking points," Ryan told Fox News.

Kevin Hassett, the newly confirmed chair of Trump's Council of Economic Advisors, even attacked the TPC analysis at an event hosted by the center on Thursday.

"I'm sure many people in these halls have been struck and perhaps even dismayed by the tone and pushback from around Washington regarding the Tax Policy Center report," Hassett said. "I think that's what happens when you behave irresponsibly."

Hassett also said the report contained "imagined numbers" and said he did not "understand the purpose of the document."

Senate Majority Leader Mitch McConnell's office dedicated a page on the leader's website to attacking the report, citing editorials from various news outlets and stories from years past that attacked TPC.

The other leading tax writer in the Senate, Finance Committee Chair Orrin Hatch, also took issue with the report, noting it contained a disclaimer that it expressed the views of the author.

"Even more unusual, no specific authors were listed on the analysis, probably because no respectable academic or researcher was willing to have their name associated with something so haphazardly cobbled together," Hatch said.

In an interview with Business Insider on Wednesday, the Tax Policy Center's Mark Mazur defended the study. He said there was public interest in analyzing the framework and all the assumptions about the plan were based on previous GOP tax reform proposals.

"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."