As part of his deal to become House speaker, Kevin McCarthy reportedly promised his party's conservative hardliners a vote on legislation that would scrap the entire American tax code and replace it with a jumbo-sized national sales tax.

The assurance got relatively little attention at the time, drowned out by the many other concessions McCarthy made to win his gavel. But with Democrats already attacking the proposal, some conservatives see it as a political headache in the making.

"This is a political gift to Biden and the Democrats," Grover Norquist, the dean of D.C. anti-tax activists, said in an interview. "I think that this is the first significant problem created for the Republican Party by the 20 people who thought that there was no downside to the approach they took."