The Tax Policy Center was the focus of Republican abuse on Tuesday after it published an unflattering analysis of Republicans' tax reform framework Friday.
House Speaker Paul Ryan called the nonprofit group "an anti-reform propaganda group."
"They're literally making up details" about the GOP plan to produce anti-reform "talking points," he said in an interview on Fox News.
Ways and Means Committee chairman Kevin Brady called the group's assessment of the plan "baloney" on Tuesday.
"They're that stuck in that liberal mindset," the Texas Republican said in an interview on Fox News Radio. Previously, he had called the analysis "a work of fiction that Stephen King would have been proud of."
On Monday morning, Senate Majority Leader Mitch McConnell's office circulated a round-up of outside criticisms of the officially nonpartisan think tank, and linked to negative articles published by the Wall Street Journal's editorial page, National Review, and others.
Without any assessment of the GOP tax framework from Congress' own budget experts or other outside organizations, the Tax Policy Center's quick analysis filled the void. Democrats and Republican Rand Paul picked up the group's statistics to attack the Republican plan.
Among the group's more damaging findings was that the framework would increase the deficit by $2.4 trillion over 10 years, provide major cuts to high income earners, and raise taxes on some households on every point of the income spectrum.
Republicans faulted the Tax Policy Center for reaching those conclusions based on an outline of the plan, without having critical details like the income levels corresponding to the three planned tax brackets, or the size of the child tax credit.
Mark Mazur, the Robert C. Pozen Director of the Tax Policy Center, brushed off the GOP criticism.
"Our mission at TPC is to provide high quality information and analysis," Mazur said. "I don't think there's any slant to it."
Mazur, who joined the group this year after serving as assistant secretary for tax policy at the Treasury under President Obama, also explained that the TPC would incorporate any further details Republicans gave them.
"We stand ready to revise it when there are details that change the analysis that are forthcoming," he said of the analysis.
Tussling with Republicans is nothing new for the Tax Policy Center. Most prominently, its analysis of Mitt Romney's presidential campaign tax plan in 2012 provided a major talking point for President Obama, as it found that Romney's plan would have meant tax hikes on middle-class families. Then, as now, Republicans accused it of partisanship, and Romney's campaign said it used a "garbage-in, garbage-out" methodology.