A failure to pass tax reform would be disastrous for the Republican Party, according to the conservative group FreedomWorks.

“If the Republicans can’t get this tax deal done, we’ll be facing a GOP crisis,” Adam Brandon, the group's president, said on a Friday call with reporters. “It could be the end of the GOP as we know it.”

The grassroots fervor that helped put Republicans in the White House and both chambers of Congress, Brandon said, was fueled by an economic anxiety that tax reform could address. Republican voters are also “extremely upset” at the party's failure, thus far, to repeal ObamaCare.

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That anger over health care, said Steve Moore, who helped advise President Trump on economic policy during his campaign, makes tax reform even more important.

“On the politics of this, Republicans are making a catastrophic mistake if they do not get this done this year,” Moore said. Failure to do so, he added, would mean that voters would not feel the reform's economic effects by the 2018 midterm elections.

Republicans are expected to roll out more details on their tax reform agenda next week, but internal disagreements remain on how to move forward.

Last week, Trump said that “the rich will not be gaining at all” under the tax plan, and might even pay more.

Rep. David Schweikert David SchweikertHouse Democratic campaign leader predicts bigger majority Democrat Hiral Tipirneni wins Ariz. primary to challenge Rep. David Schweikert Ethics watchdog finds 'substantial' evidence of improper spending by Rep. Sanford Bishop MORE (R-Ariz.), a member of the House Ways and Means Committee that will be central to writing the tax plan, wouldn't go that far.

“I’m not saying that, I’m saying that they will not benefit the same as the working, middle class,” Schweikert said. He noted, however, that tackling deductions that benefit the wealthy would partially counter the effects of lowering their tax rates.

Schweikert is a member of the conservative House Freedom Caucus.